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