tz-how-to.html revision 1.9
1 1.8 christos <!DOCTYPE html> 2 1.8 christos <html lang="en"> 3 1.3 christos <head> 4 1.3 christos <title>How to Read the tz Database</title> 5 1.8 christos <meta charset="UTF-8"> 6 1.8 christos <style> 7 1.5 christos pre {margin-left: 2em; white-space: pre-wrap;} 8 1.8 christos pre.td {margin-left: 0;} 9 1.8 christos td {text-align: center;} 10 1.8 christos table {border: 1px outset;} 11 1.8 christos th, td {border: 1px inset;} 12 1.8 christos table.rule {border: none; margin: auto;} 13 1.8 christos td.footnote {text-align: left;} 14 1.5 christos </style> 15 1.3 christos </head> 16 1.1 christos <body> 17 1.1 christos <h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 18 1.1 christos Database</a> Source Files</h2> 19 1.1 christos <h3>by Bill Seymour</h3> 20 1.9 christos <p>This guide uses the <code>America/Chicago</code> and 21 1.1 christos <code>Pacific/Honolulu</code> zones as examples of how to infer 22 1.4 christos times of day from the <a href="tz-link.html">tz database</a> 23 1.1 christos source files. It might be helpful, but not absolutely necessary, 24 1.1 christos for the reader to have already downloaded the 25 1.2 christos latest release of the database and become familiar with the basic layout 26 1.1 christos of the data files. The format is explained in the “man 27 1.1 christos page” for the zic compiler, <code>zic.8.txt</code>, in 28 1.9 christos the <code>code</code> subdirectory. 29 1.9 christos Although this guide covers many of the common cases, it is not a 30 1.9 christos complete summary of what zic accepts; the man page is the 31 1.9 christos authoritative reference.</p> 32 1.1 christos 33 1.1 christos <p>We’ll begin by talking about the rules for changing between standard 34 1.1 christos and daylight saving time since we’ll need that information when we talk 35 1.1 christos about the zones.</p> 36 1.1 christos 37 1.1 christos <p>First, let’s consider the special daylight saving time rules 38 1.1 christos for Chicago (from the <code>northamerica</code> file in 39 1.1 christos the <code>data</code> subdirectory):</p> 40 1.1 christos 41 1.8 christos <table> 42 1.1 christos <tr> 43 1.1 christos <th colspan="6">From the Source File</th> 44 1.1 christos </tr> 45 1.1 christos <tr> 46 1.8 christos <td colspan="6"> 47 1.8 christos <table class="rule"> 48 1.8 christos <tr><td style="border:none;text-align:left"> 49 1.8 christos <pre class="td"> 50 1.8 christos #Rule NAME FROM TO - IN ON AT SAVE LETTER 51 1.1 christos Rule Chicago 1920 only - Jun 13 2:00 1:00 D 52 1.1 christos Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 53 1.1 christos Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 54 1.1 christos Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 55 1.1 christos Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 56 1.1 christos Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S 57 1.1 christos </pre> 58 1.1 christos </td></tr></table></td> 59 1.1 christos </tr> 60 1.1 christos <tr> 61 1.1 christos <th colspan="6">Reformatted a Bit</th> 62 1.1 christos </tr> 63 1.1 christos <tr> 64 1.1 christos <th>From</th> 65 1.1 christos <th>To</th> 66 1.1 christos <th colspan="2">On</th> 67 1.1 christos <th>At</th> 68 1.1 christos <th>Action</th> 69 1.1 christos </tr> 70 1.8 christos <tr> 71 1.1 christos <td colspan="2">1920 only</td> 72 1.1 christos <td colspan="2">June 13<small><sup>th</sup></small></td> 73 1.1 christos <td rowspan="6">02:00 local</td> 74 1.1 christos <td>go to daylight saving time</td> 75 1.1 christos </tr> 76 1.8 christos <tr> 77 1.1 christos <td>1920</td> 78 1.1 christos <td>1921</td> 79 1.1 christos <td rowspan="5">last Sunday</td> 80 1.1 christos <td>in October</td> 81 1.1 christos <td>return to standard time</td> 82 1.1 christos </tr> 83 1.8 christos <tr> 84 1.1 christos <td colspan="2">1921 only</td> 85 1.1 christos <td>in March</td> 86 1.1 christos <td rowspan="2">go to daylight saving time</td> 87 1.1 christos </tr> 88 1.8 christos <tr> 89 1.1 christos <td rowspan="2">1922</td> 90 1.1 christos <td>1966</td> 91 1.1 christos <td>in April</td> 92 1.1 christos </tr> 93 1.8 christos <tr> 94 1.1 christos <td>1954</td> 95 1.1 christos <td>in September</td> 96 1.1 christos <td rowspan="2">return to standard time</td> 97 1.1 christos </tr> 98 1.8 christos <tr> 99 1.1 christos <td>1955</td> 100 1.1 christos <td>1966</td> 101 1.1 christos <td>in October</td> 102 1.1 christos </tr> 103 1.1 christos </table> 104 1.1 christos 105 1.8 christos <p>The <code>FROM</code> and <code>TO</code> columns, respectively, specify the 106 1.8 christos first and last calendar years defining a contiguous range over which a specific 107 1.8 christos Rule line is to apply. The keyword <code>only</code> can be used in the 108 1.8 christos <code>TO</code> field to repeat the value of the <code>FROM</code> field in the 109 1.8 christos event that a rule should only apply to a single year. Often, the keyword 110 1.8 christos <code>max</code> is used to extend a rule’s application into the 111 1.8 christos indefinite future; it is a platform-agnostic stand-in for the largest 112 1.8 christos representable year. 113 1.8 christos 114 1.8 christos <p>The next column, <code>-</code>, is reserved; for compatibility with earlier 115 1.8 christos releases, it always contains a hyphen, which acts as a kind of null value. 116 1.8 christos Prior to the 2020b release, it was called the <code>TYPE</code> field, though 117 1.9 christos it had not been used in the main data since the 2000e release. 118 1.9 christos An obsolescent supplementary file used the 119 1.8 christos field as a proof-of-concept to allow <code>zic</code> to apply a given Rule 120 1.8 christos line only to certain “types” of years within the specified range as 121 1.8 christos dictated by the output of a separate script, such as: only years which would 122 1.8 christos have a US presidential election, or only years which wouldn’t. 123 1.1 christos 124 1.7 christos <p>The <code>SAVE</code> column contains the local (wall clock) offset from 125 1.1 christos local standard time. 126 1.1 christos This is usually either zero for standard time or one hour for daylight 127 1.1 christos saving time; but there’s no reason, in principle, why it can’t 128 1.1 christos take on other values. 129 1.1 christos 130 1.1 christos <p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 131 1.1 christos column can contain a variable 132 1.1 christos part of the usual abbreviation of the time zone’s name, or it can just 133 1.1 christos be a hyphen if there’s no variable part. For example, the abbreviation 134 1.1 christos used in the central time zone will be either “CST” or 135 1.1 christos “CDT”. The variable part is ‘S’ or ‘D’; 136 1.1 christos and, sure enough, that’s just what we find in 137 1.1 christos the <code>LETTER</code> column 138 1.1 christos in the <code>Chicago</code> rules. More about this when we talk about 139 1.1 christos “Zone” lines. 140 1.1 christos 141 1.1 christos <p>One important thing to notice is that “Rule” lines 142 1.1 christos want at once to be both <i>transitions</i> and <i>steady states</i>: 143 1.1 christos <ul> 144 1.1 christos <li>On the one hand, they represent transitions between standard and 145 1.1 christos daylight saving time; and any number of Rule lines can be in effect 146 1.1 christos during a given period (which will always be a non-empty set of 147 1.1 christos contiguous calendar years).</li> 148 1.1 christos <li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 149 1.1 christos columns contain state that exists between transitions. More about this 150 1.1 christos when we talk about the US rules.</li> 151 1.1 christos </ul> 152 1.1 christos 153 1.1 christos <p>In the example above, the transition to daylight saving time 154 1.1 christos happened on the 13<small><sup>th</sup></small> of June in 1920, and on 155 1.1 christos the last Sunday in March in 1921; but the return to standard time 156 1.1 christos happened on the last Sunday in October in both of those 157 1.1 christos years. Similarly, the rule for changing to daylight saving time was 158 1.1 christos the same from 1922 to 1966; but the rule for returning to standard 159 1.1 christos time changed in 1955. Got it?</p> 160 1.1 christos 161 1.1 christos <p>OK, now for the somewhat more interesting “US” rules:</p> 162 1.1 christos 163 1.8 christos <table> 164 1.1 christos <tr> 165 1.1 christos <th colspan="6">From the Source File</th> 166 1.1 christos </tr> 167 1.1 christos <tr> 168 1.8 christos <td colspan="6"> 169 1.8 christos <table class="rule"> 170 1.8 christos <tr><td style="border:none;text-align:left"> 171 1.8 christos <pre class="td"> 172 1.8 christos #Rule NAME FROM TO - IN ON AT SAVE LETTER/S 173 1.1 christos Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 174 1.1 christos Rule US 1918 1919 - Oct lastSun 2:00 0 S 175 1.1 christos Rule US 1942 only - Feb 9 2:00 1:00 W # War 176 1.1 christos Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 177 1.1 christos Rule US 1945 only - Sep 30 2:00 0 S 178 1.1 christos Rule US 1967 2006 - Oct lastSun 2:00 0 S 179 1.1 christos Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 180 1.1 christos Rule US 1974 only - Jan 6 2:00 1:00 D 181 1.1 christos Rule US 1975 only - Feb 23 2:00 1:00 D 182 1.1 christos Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 183 1.1 christos Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 184 1.1 christos Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 185 1.1 christos Rule US 2007 max - Nov Sun>=1 2:00 0 S 186 1.1 christos </pre> 187 1.1 christos </td></tr></table></td> 188 1.1 christos </tr> 189 1.1 christos <tr> 190 1.1 christos <th colspan="6">Reformatted a Bit</th> 191 1.1 christos </tr> 192 1.1 christos <tr> 193 1.1 christos <th>From</th> 194 1.1 christos <th>To</th> 195 1.1 christos <th colspan="2">On</th> 196 1.1 christos <th>At</th> 197 1.1 christos <th>Action</th> 198 1.1 christos </tr> 199 1.8 christos <tr> 200 1.1 christos <td rowspan="2">1918</td> 201 1.1 christos <td rowspan="2">1919</td> 202 1.1 christos <td rowspan="2">last Sunday</td> 203 1.1 christos <td>in March</td> 204 1.1 christos <td rowspan="3">02:00 local</td> 205 1.1 christos <td>go to daylight saving time</td> 206 1.1 christos </tr> 207 1.8 christos <tr> 208 1.1 christos <td>in October</td> 209 1.1 christos <td>return to standard time</td> 210 1.1 christos </tr> 211 1.8 christos <tr> 212 1.1 christos <td colspan="2">1942 only</td> 213 1.1 christos <td colspan="2">February 9<small><sup>th</sup></small></td> 214 1.1 christos <td>go to “war time”</td> 215 1.1 christos </tr> 216 1.8 christos <tr> 217 1.1 christos <td colspan="2" rowspan="2">1945 only</td> 218 1.1 christos <td colspan="2">August 14<small><sup>th</sup></small></td> 219 1.1 christos <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 220 1.1 christos <td> 221 1.1 christos rename “war time” to “peace<br>time;” 222 1.1 christos clocks don’t change 223 1.1 christos </td> 224 1.1 christos </tr> 225 1.8 christos <tr> 226 1.1 christos <td colspan="2">September 30<small><sup>th</sup></small></td> 227 1.1 christos <td rowspan="9">02:00 local</td> 228 1.1 christos <td rowspan="2">return to standard time</td> 229 1.1 christos </tr> 230 1.8 christos <tr> 231 1.1 christos <td rowspan="2">1967</td> 232 1.1 christos <td>2006</td> 233 1.1 christos <td rowspan="2">last Sunday</td> 234 1.1 christos <td>in October</td> 235 1.1 christos </tr> 236 1.8 christos <tr> 237 1.1 christos <td>1973</td> 238 1.1 christos <td>in April</td> 239 1.1 christos <td rowspan="6">go to daylight saving time</td> 240 1.1 christos </tr> 241 1.8 christos <tr> 242 1.1 christos <td colspan="2">1974 only</td> 243 1.1 christos <td colspan="2">January 6<small><sup>th</sup></small></td> 244 1.1 christos </tr> 245 1.8 christos <tr> 246 1.1 christos <td colspan="2">1975 only</td> 247 1.1 christos <td colspan="2">February 23<small><sup>rd</sup></small></td> 248 1.1 christos </tr> 249 1.8 christos <tr> 250 1.1 christos <td>1976</td> 251 1.1 christos <td>1986</td> 252 1.1 christos <td>last Sunday</td> 253 1.1 christos <td rowspan="2">in April</td> 254 1.1 christos </tr> 255 1.8 christos <tr> 256 1.1 christos <td>1987</td> 257 1.1 christos <td>2006</td> 258 1.1 christos <td>first Sunday</td> 259 1.1 christos </tr> 260 1.8 christos <tr> 261 1.1 christos <td rowspan="2">2007</td> 262 1.1 christos <td rowspan="2">present</td> 263 1.1 christos <td colspan="2">second Sunday in March</td> 264 1.1 christos </tr> 265 1.8 christos <tr> 266 1.1 christos <td colspan="2">first Sunday in November</td> 267 1.1 christos <td>return to standard time</td> 268 1.1 christos </tr> 269 1.1 christos </table> 270 1.1 christos 271 1.1 christos <p>There are two interesting things to note here.</p> 272 1.1 christos 273 1.1 christos <p>First, the time that something happens (in the <code>AT</code> 274 1.7 christos column) is not necessarily the local (wall clock) time. The time can be 275 1.1 christos suffixed with ‘s’ (for “standard”) to mean 276 1.7 christos local standard time, different from local (wall clock) time when observing 277 1.7 christos daylight saving time; or it can be suffixed with ‘g’, 278 1.1 christos ‘u’, or ‘z’, all three of which mean the 279 1.1 christos standard time at the 280 1.2 christos <a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 281 1.1 christos ‘g’ stands for “<a 282 1.1 christos href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 283 1.1 christos ‘u’ stands for “<a 284 1.1 christos href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 285 1.1 christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 286 1.1 christos (whichever was official at the time); ‘z’ stands for the 287 1.1 christos <a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 288 1.1 christos Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 289 1.7 christos The time can also be suffixed with ‘w’ meaning local (wall 290 1.7 christos clock) time; but it usually isn’t because that’s the 291 1.1 christos default.</p> 292 1.1 christos 293 1.1 christos <p>Second, the day in the <code>ON</code> column, in addition to 294 1.1 christos “<code>lastSun</code>” or a particular day of the month, 295 1.1 christos can have the form, “<code>Sun>=</code><i>x</i>” or 296 1.1 christos “<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 297 1.1 christos of the month. For example, “<code>Sun>=8</code>” means 298 1.1 christos “the first Sunday on or after the eighth of the month,” in 299 1.1 christos other words, the second Sunday of the month. Furthermore, although 300 1.1 christos there are no examples above, the weekday needn’t be 301 1.1 christos “<code>Sun</code>” in either form, but can be the usual 302 1.1 christos three-character English abbreviation for any day of the week.</p> 303 1.1 christos 304 1.1 christos <p>And the US rules give us more examples of a couple of things 305 1.1 christos already mentioned:</p> 306 1.1 christos 307 1.1 christos <ul> 308 1.1 christos <li>The rules for changing to and from daylight saving time are 309 1.1 christos actually <i>different sets</i> of rules; and the two sets can change 310 1.1 christos independently. Consider, for example, that the rule for the return to 311 1.1 christos standard time stayed the same from 1967 to 2006; but the rule for the 312 1.1 christos transition to daylight saving time changed several times in the same 313 1.1 christos period. There can also be periods, 1946 to 1966 for example, when no 314 1.1 christos rule from this group is in effect, and so either no transition 315 1.1 christos happened in those years, or some other rule is in effect (perhaps a 316 1.1 christos state or other more local rule).</li> 317 1.1 christos 318 1.1 christos <li>The <code>SAVE</code> and <code>LETTER</code> columns 319 1.1 christos contain <i>steady state</i>, not transitions. Consider, for example, 320 1.1 christos the transition from “war time” to “peace time” 321 1.1 christos that happened on August 14, 1945. The “1:00” in 322 1.1 christos the <code>SAVE</code> column is <i>not</i> an instruction to advance 323 1.1 christos the clock an hour. It means that clocks should <i>be</i> one hour 324 1.1 christos ahead of standard time, which they already are because of the previous 325 1.1 christos rule, so there should be no change.</li> 326 1.1 christos 327 1.1 christos </ul> 328 1.1 christos 329 1.1 christos <p>OK, now let’s look at a Zone record:</p> 330 1.1 christos 331 1.8 christos <table> 332 1.1 christos <tr> 333 1.1 christos <th colspan="5">From the Source File</th> 334 1.1 christos </tr> 335 1.1 christos <tr> 336 1.8 christos <td colspan="5"> 337 1.8 christos <table class="rule"> 338 1.8 christos <tr><td style="border:none;text-align:left"> 339 1.8 christos <pre class="td"> 340 1.7 christos #Zone NAME STDOFF RULES FORMAT [UNTIL] 341 1.1 christos Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 342 1.1 christos -6:00 US C%sT 1920 343 1.1 christos -6:00 Chicago C%sT 1936 Mar 1 2:00 344 1.1 christos -5:00 - EST 1936 Nov 15 2:00 345 1.1 christos -6:00 Chicago C%sT 1942 346 1.1 christos -6:00 US C%sT 1946 347 1.1 christos -6:00 Chicago C%sT 1967 348 1.1 christos -6:00 US C%sT 349 1.1 christos </pre> 350 1.1 christos </td></tr></table></td> 351 1.1 christos </tr> 352 1.1 christos <tr> 353 1.1 christos <th colspan="5">Columns Renamed</th> 354 1.1 christos </tr> 355 1.1 christos <tr> 356 1.1 christos <th rowspan="2">Standard Offset<br> 357 1.1 christos from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 358 1.1 christos Meridian</a></th> 359 1.1 christos <th rowspan="2">Daylight<br>Saving Time</th> 360 1.1 christos <th rowspan="2">Abbreviation(s)</th> 361 1.1 christos <th colspan="2">Ending at Local Time</th> 362 1.1 christos </tr> 363 1.1 christos <tr> 364 1.1 christos <th>Date</th> 365 1.1 christos <th>Time</th> 366 1.1 christos </tr> 367 1.8 christos <tr> 368 1.1 christos <td>−5:50:36</td> 369 1.1 christos <td>not observed</td> 370 1.1 christos <td>LMT</td> 371 1.1 christos <td>1883-11-18</td> 372 1.1 christos <td>12:09:24</td> 373 1.1 christos </tr> 374 1.8 christos <tr> 375 1.1 christos <td rowspan="2">−6:00:00</td> 376 1.1 christos <td>US rules</td> 377 1.1 christos <td rowspan="2">CST or CDT</td> 378 1.1 christos <td>1920-01-01</td> 379 1.1 christos <td>00:00:00</td> 380 1.1 christos </tr> 381 1.8 christos <tr> 382 1.1 christos <td>Chicago rules</td> 383 1.1 christos <td>1936-03-01</td> 384 1.1 christos <td rowspan="2">02:00:00</td> 385 1.1 christos </tr> 386 1.8 christos <tr> 387 1.1 christos <td>−5:00:00</td> 388 1.1 christos <td>not observed</td> 389 1.1 christos <td>EST</td> 390 1.1 christos <td>1936-11-15</td> 391 1.1 christos </tr> 392 1.8 christos <tr> 393 1.1 christos <td rowspan="4">−6:00:00</td> 394 1.1 christos <td>Chicago rules</td> 395 1.1 christos <td>CST or CDT</td> 396 1.1 christos <td>1942-01-01</td> 397 1.1 christos <td rowspan="3">00:00:00</td> 398 1.1 christos </tr> 399 1.8 christos <tr> 400 1.1 christos <td>US rules</td> 401 1.1 christos <td>CST, CWT or CPT</td> 402 1.1 christos <td>1946-01-01</td> 403 1.1 christos </tr> 404 1.8 christos <tr> 405 1.1 christos <td>Chicago rules</td> 406 1.1 christos <td rowspan="2">CST or CDT</td> 407 1.1 christos <td>1967-01-01</td> 408 1.1 christos </tr> 409 1.8 christos <tr> 410 1.1 christos <td>US rules</td> 411 1.1 christos <td colspan="2">—</td> 412 1.1 christos </tr> 413 1.1 christos </table> 414 1.1 christos 415 1.1 christos <p>There are a couple of interesting differences between Zones and Rules.</p> 416 1.1 christos 417 1.1 christos <p>First, and somewhat trivially, whereas Rules are considered to 418 1.1 christos contain one or more records, a Zone is considered to be a single 419 1.1 christos record with zero or more <i>continuation lines</i>. Thus, the keyword, 420 1.1 christos “<code>Zone</code>,” and the zone name are not 421 1.1 christos repeated. The last line is the one without anything in 422 1.1 christos the <code>[UNTIL]</code> column.</p> 423 1.1 christos 424 1.1 christos <p>Second, and more fundamentally, each line of a Zone represents a 425 1.1 christos steady state, not a transition between states. The state exists from 426 1.1 christos the date and time in the previous line’s <code>[UNTIL]</code> 427 1.1 christos column up to the date and time in the current 428 1.1 christos line’s <code>[UNTIL]</code> column. In other words, the date and 429 1.1 christos time in the <code>[UNTIL]</code> column is the instant that separates 430 1.1 christos this state from the next. Where that would be ambiguous because 431 1.1 christos we’re setting our clocks back, the <code>[UNTIL]</code> column 432 1.1 christos specifies the first occurrence of the instant. The state specified by 433 1.1 christos the last line, the one without anything in the <code>[UNTIL]</code> 434 1.1 christos column, continues to the present.</p> 435 1.1 christos 436 1.1 christos <p>The first line typically specifies the mean solar time observed 437 1.1 christos before the introduction of standard time. Since there’s no line before 438 1.1 christos that, it has no beginning. <code>8-) </code> For some places near the <a 439 1.1 christos href="https://en.wikipedia.org/wiki/International_Date_Line">International 440 1.1 christos Date Line</a>, the first <i>two</i> lines will show solar times 441 1.1 christos differing by 24 hours; this corresponds to a movement of the Date 442 1.1 christos Line. For example:</p> 443 1.1 christos 444 1.1 christos <pre> 445 1.7 christos #Zone NAME STDOFF RULES FORMAT [UNTIL] 446 1.1 christos Zone America/Juneau 15:02:19 - LMT 1867 Oct 18 447 1.1 christos -8:57:41 - LMT ... 448 1.1 christos </pre> 449 1.1 christos 450 1.1 christos <p>When Alaska was purchased from Russia in 1867, the Date Line moved 451 1.1 christos from the Alaska/Canada border to the Bering Strait; and the time in 452 1.1 christos Alaska was then 24 hours earlier than it had 453 1.1 christos been. <code><aside></code>(6 October in the Julian calendar, 454 1.1 christos which Russia was still using then for religious reasons, was followed 455 1.1 christos by <i>a second instance of the same day with a different name</i>, 18 456 1.1 christos October in the Gregorian calendar. Isn’t civil time 457 1.1 christos wonderful? <code>8-)</code>)<code></aside></code></p> 458 1.1 christos 459 1.1 christos <p>The abbreviation, “LMT” stands for “local mean 460 1.1 christos time”, which is an invention of 461 1.1 christos the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 462 1.1 christos database</a> and was probably never actually used during the 463 1.1 christos period. Furthermore, the value is almost certainly wrong except in the 464 1.1 christos archetypal place after which the zone is named. (The tz database 465 1.1 christos usually doesn’t provide a separate Zone record for places where 466 1.1 christos nothing significant happened after 1970.)</p> 467 1.1 christos 468 1.1 christos <p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 469 1.1 christos <ul> 470 1.1 christos <li>A hyphen, a kind of null value, means that we have not set our 471 1.1 christos clocks ahead of standard time.</li> 472 1.1 christos 473 1.1 christos <li>An amount of time (usually but not necessarily “1:00” 474 1.1 christos meaning one hour) means that we have set our clocks ahead by that 475 1.1 christos amount.</li> 476 1.1 christos 477 1.1 christos <li>Some alphabetic string means that we <i>might have</i> set our 478 1.1 christos clocks ahead; and we need to check the rule the name of which is the 479 1.1 christos given alphabetic string.</li> 480 1.1 christos </ul> 481 1.1 christos 482 1.1 christos <p>An example of a specific amount of time is:</p> 483 1.1 christos <pre> 484 1.7 christos #Zone NAME STDOFF RULES FORMAT [UNTIL] 485 1.6 christos Zone Pacific/Honolulu ... 1933 Apr 30 2:00 486 1.6 christos -10:30 1:00 HDT 1933 May 21 12:00 487 1.1 christos ... 488 1.1 christos </pre> 489 1.1 christos 490 1.1 christos <p>Hawaii tried daylight saving time for three weeks in 1933 and 491 1.1 christos decided they didn’t like it. <code>8-) </code>Note that 492 1.7 christos the <code>STDOFF</code> column always contains the standard time 493 1.7 christos offset, so the local (wall clock) time during this period was GMT − 494 1.1 christos 10:30 + 1:00 = GMT − 9:30.</p> 495 1.1 christos 496 1.1 christos <p>The <code>FORMAT</code> column specifies the usual abbreviation of 497 1.9 christos the time zone name. It should have one of four forms:</p> 498 1.1 christos <ul> 499 1.1 christos 500 1.9 christos <li>a time zone abbreviation that is a string of three or more 501 1.9 christos characters that are either ASCII alphanumerics, 502 1.9 christos “<code>+</code>”, or “<code>-</code>”</li> 503 1.9 christos 504 1.9 christos <li>the string “%z”, in which case the 505 1.9 christos “<code>%z</code>” will be replaced by a numeric time zone 506 1.9 christos abbreviation</li> 507 1.1 christos 508 1.9 christos <li>a pair of time zone abbreviations separated by a slash 509 1.1 christos (‘<code>/</code>’), in which case the first string is the 510 1.1 christos abbreviation for the standard time name and the second string is the 511 1.1 christos abbreviation for the daylight saving time name</li> 512 1.1 christos 513 1.9 christos <li>a string containing “<code>%s</code>”, in which case 514 1.1 christos the “<code>%s</code>” will be replaced by the text in the 515 1.9 christos appropriate Rule’s <code>LETTER</code> column, and the resulting 516 1.9 christos string should be a time zone abbreviation</li> 517 1.1 christos </ul> 518 1.1 christos 519 1.1 christos <p>The last two make sense only if there’s a named rule in effect.</p> 520 1.1 christos 521 1.1 christos <p>An example of a slash is:</p> 522 1.1 christos <pre> 523 1.7 christos #Zone NAME STDOFF RULES FORMAT [UNTIL] 524 1.1 christos Zone Europe/London ... 1996 525 1.1 christos 0:00 EU GMT/BST 526 1.1 christos </pre> 527 1.1 christos 528 1.1 christos <p>The current time in the UK is called either Greenwich mean time or 529 1.1 christos British summer time.</p> 530 1.1 christos 531 1.1 christos <p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 532 1.1 christos happens when switching to a named rule. To what values should 533 1.1 christos the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 534 1.1 christos 535 1.1 christos <ul> 536 1.1 christos <li>If at least one transition has happened, use 537 1.1 christos the <code>SAVE</code> and <code>LETTER</code> data from the most 538 1.1 christos recent.</li> 539 1.1 christos 540 1.1 christos <li>If switching to a named rule before any transition has happened, 541 1.1 christos assume standard time (<code>SAVE</code> zero), and use 542 1.1 christos the <code>LETTER</code> data from the earliest transition with 543 1.1 christos a <code>SAVE</code> of zero. 544 1.1 christos 545 1.1 christos </ul> 546 1.1 christos 547 1.1 christos <p>And three last things about the <code>FORMAT</code> column:</p> 548 1.1 christos <ul> 549 1.1 christos 550 1.1 christos <li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 551 1.5 christos database</a> gives abbreviations for time zones in <i>popular 552 1.1 christos usage</i>, which is not necessarily “correct” by law. For 553 1.1 christos example, the last line in 554 1.1 christos <code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 555 1.1 christos “HST” for “Hawaii standard time” even though the 556 1.3 christos <a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a> 557 1.1 christos name for that time zone is “Hawaii-Aleutian standard time.” 558 1.1 christos This author has read that there are also some places in Australia where 559 1.1 christos popular time zone names differ from the legal ones. 560 1.1 christos 561 1.1 christos <li>No attempt is made to <a 562 1.1 christos href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 563 1.1 christos the abbreviations. They are intended to be the values returned through the 564 1.1 christos <code>"%Z"</code> format specifier to 565 1.1 christos <a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 566 1.7 christos <a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 567 1.1 christos function in the 568 1.7 christos <a href="https://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 569 1.1 christos 570 1.1 christos <li>If there is no generally-accepted abbreviation for a time zone, 571 1.1 christos a numeric offset is used instead, e.g., <code>+07</code> for 7 hours 572 1.1 christos ahead of Greenwich. By convention, <code>-00</code> is used in a 573 1.1 christos zone while uninhabited, where the offset is zero but in some sense 574 1.1 christos the true offset is undefined. 575 1.1 christos </ul> 576 1.1 christos 577 1.1 christos <p>As a final example, here’s the complete history for Hawaii:</p> 578 1.1 christos 579 1.8 christos <table> 580 1.1 christos <tr> 581 1.1 christos <th colspan="6">Relevant Excerpts from the US Rules</th> 582 1.1 christos </tr> 583 1.1 christos <tr> 584 1.8 christos <td colspan="6"> 585 1.8 christos <table class="rule"> 586 1.8 christos <tr><td style="border:none;text-align:left"> 587 1.8 christos <pre class="td"> 588 1.8 christos #Rule NAME FROM TO - IN ON AT SAVE LETTER/S 589 1.1 christos Rule US 1918 1919 - Oct lastSun 2:00 0 S 590 1.1 christos Rule US 1942 only - Feb 9 2:00 1:00 W # War 591 1.1 christos Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 592 1.6 christos Rule US 1945 only - Sep lastSun 2:00 0 S 593 1.1 christos </pre> 594 1.1 christos </td></tr></table></td> 595 1.1 christos </tr> 596 1.1 christos <tr> 597 1.1 christos <th colspan="6">The Zone Record</th> 598 1.1 christos </tr> 599 1.1 christos <tr> 600 1.8 christos <td colspan="6"> 601 1.8 christos <table class="rule"> 602 1.8 christos <tr><td style="border:none;text-align:left"> 603 1.8 christos <pre class="td"> 604 1.7 christos #Zone NAME STDOFF RULES FORMAT [UNTIL] 605 1.6 christos Zone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 606 1.1 christos -10:30 - HST 1933 Apr 30 2:00 607 1.1 christos -10:30 1:00 HDT 1933 May 21 2:00 608 1.1 christos -10:30 US H%sT 1947 Jun 8 2:00 609 1.1 christos -10:00 - HST 610 1.1 christos </pre> 611 1.1 christos </td></tr></table></td> 612 1.1 christos </tr> 613 1.1 christos <tr> 614 1.1 christos <th colspan="6">What We Infer</th> 615 1.1 christos </tr> 616 1.1 christos <tr> 617 1.1 christos <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 618 1.1 christos <th rowspan="2">Adjust<br>Clocks</th> 619 1.1 christos <th colspan="2">Time Zone</th> 620 1.1 christos <th colspan="2">Ending at Local Time</th> 621 1.1 christos </tr> 622 1.1 christos <tr> 623 1.1 christos <th>Abbrv.</th> 624 1.1 christos <th>Name</th> 625 1.1 christos <th>Date</th> 626 1.1 christos <th>Time</th> 627 1.1 christos </tr> 628 1.8 christos <tr> 629 1.1 christos <td>−10:31:26</td> 630 1.1 christos <td>—</td> 631 1.1 christos <td>LMT</td> 632 1.1 christos <td>local mean time</td> 633 1.6 christos <td>1896-01-13</td> 634 1.1 christos <td>12:00</td> 635 1.1 christos </tr> 636 1.8 christos <tr> 637 1.1 christos <td>−10:30</td> 638 1.1 christos <td>+0:01:26</td> 639 1.1 christos <td>HST</td> 640 1.1 christos <td>Hawaii standard time</td> 641 1.1 christos <td>1933-04-30</td> 642 1.6 christos <td>02:00</td> 643 1.1 christos </tr> 644 1.8 christos <tr> 645 1.1 christos <td>−9:30</td> 646 1.1 christos <td>+1:00</td> 647 1.1 christos <td>HDT</td> 648 1.1 christos <td>Hawaii daylight time</td> 649 1.1 christos <td>1933-05-21</td> 650 1.6 christos <td>12:00</td> 651 1.1 christos </tr> 652 1.8 christos <tr> 653 1.1 christos <td>−10:30¹</td> 654 1.1 christos <td>−1:00¹</td> 655 1.1 christos <td>HST¹</td> 656 1.1 christos <td>Hawaii standard time</td> 657 1.1 christos <td>1942-02-09</td> 658 1.6 christos <td>02:00</td> 659 1.1 christos </tr> 660 1.8 christos <tr> 661 1.1 christos <td rowspan="2">−9:30</td> 662 1.1 christos <td>+1:00</td> 663 1.1 christos <td>HWT</td> 664 1.1 christos <td>Hawaii war time</td> 665 1.1 christos <td>1945-08-14</td> 666 1.1 christos <td>13:30²</td> 667 1.1 christos </tr> 668 1.8 christos <tr> 669 1.1 christos <td>0</td> 670 1.1 christos <td>HPT</td> 671 1.1 christos <td>Hawaii peace time</td> 672 1.1 christos <td>1945-09-30</td> 673 1.1 christos <td rowspan="2">02:00</td> 674 1.1 christos </tr> 675 1.8 christos <tr> 676 1.1 christos <td>−10:30</td> 677 1.1 christos <td>−1:00</td> 678 1.1 christos <td rowspan="2">HST</td> 679 1.1 christos <td rowspan="2">Hawaii standard time</td> 680 1.1 christos <td>1947-06-08</td> 681 1.1 christos </tr> 682 1.8 christos <tr> 683 1.1 christos <td>−10:00³</td> 684 1.1 christos <td>+0:30³</td> 685 1.1 christos <td colspan="2">—</td> 686 1.1 christos </tr> 687 1.1 christos <tr> 688 1.8 christos <td colspan="6" class="footnote"> 689 1.1 christos ¹Switching to US rules…most recent transition (in 1919) was to standard time 690 1.1 christos </td> 691 1.1 christos </tr> 692 1.1 christos <tr> 693 1.8 christos <td colspan="6" class="footnote"> 694 1.1 christos ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 695 1.1 christos + (−9:30) = 13:30 local 696 1.1 christos </td> 697 1.1 christos </tr> 698 1.1 christos <tr> 699 1.8 christos <td colspan="6" class="footnote"> 700 1.1 christos ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 701 1.1 christos the civil time in Hawaii has been 702 1.1 christos <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>/<a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a> 703 1.1 christos − 10:00 year-round. 704 1.1 christos </td> 705 1.1 christos </tr> 706 1.1 christos </table> 707 1.1 christos 708 1.1 christos <p>There will be a short quiz later. <code>8-)</code></p> 709 1.1 christos 710 1.1 christos <hr> 711 1.1 christos <address> 712 1.1 christos This web page is in the public domain, so clarified as of 713 1.1 christos 2015-10-20 by Bill Seymour. 714 1.1 christos <br> 715 1.1 christos All suggestions and corrections will be welcome; all flames will be amusing. 716 1.1 christos Mail to was at pobox dot com. 717 1.1 christos </address> 718 1.1 christos </body> 719 1.1 christos </html> 720
Indexes created Mon Sep 29 21:09:56 GMT 2025