tz-how-to.html revision 1.6
1 1.1 christos <!DOCTYPE html 2 1.1 christos PUBLIC "-//W3C//DTD HTML 4.01//EN" 3 1.1 christos "http://www.w3.org/TR/html4/strict.dtd"> 4 1.1 christos <html> 5 1.3 christos <head> 6 1.3 christos <title>How to Read the tz Database</title> 7 1.3 christos <meta http-equiv="Content-type" content='text/html; charset="UTF-8"'> 8 1.5 christos <style type="text/css"> 9 1.5 christos pre {margin-left: 2em; white-space: pre-wrap;} 10 1.5 christos </style> 11 1.3 christos </head> 12 1.1 christos <body> 13 1.1 christos <h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 14 1.1 christos Database</a> Source Files</h2> 15 1.1 christos <h3>by Bill Seymour</h3> 16 1.1 christos <p>This page uses the <code>America/Chicago</code> and 17 1.1 christos <code>Pacific/Honolulu</code> zones as examples of how to infer 18 1.4 christos times of day from the <a href="tz-link.html">tz database</a> 19 1.1 christos source files. It might be helpful, but not absolutely necessary, 20 1.1 christos for the reader to have already downloaded the 21 1.2 christos latest release of the database and become familiar with the basic layout 22 1.1 christos of the data files. The format is explained in the “man 23 1.1 christos page” for the zic compiler, <code>zic.8.txt</code>, in 24 1.1 christos the <code>code</code> subdirectory.</p> 25 1.1 christos 26 1.1 christos <p>We’ll begin by talking about the rules for changing between standard 27 1.1 christos and daylight saving time since we’ll need that information when we talk 28 1.1 christos about the zones.</p> 29 1.1 christos 30 1.1 christos <p>First, let’s consider the special daylight saving time rules 31 1.1 christos for Chicago (from the <code>northamerica</code> file in 32 1.1 christos the <code>data</code> subdirectory):</p> 33 1.1 christos 34 1.1 christos <table border="1"> 35 1.1 christos <tr> 36 1.1 christos <th colspan="6">From the Source File</th> 37 1.1 christos </tr> 38 1.1 christos <tr> 39 1.1 christos <td colspan="6" align="center"><table><tr><td> 40 1.1 christos <pre> 41 1.1 christos #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER 42 1.1 christos Rule Chicago 1920 only - Jun 13 2:00 1:00 D 43 1.1 christos Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 44 1.1 christos Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 45 1.1 christos Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 46 1.1 christos Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 47 1.1 christos Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S 48 1.1 christos </pre> 49 1.1 christos </td></tr></table></td> 50 1.1 christos </tr> 51 1.1 christos <tr> 52 1.1 christos <th colspan="6">Reformatted a Bit</th> 53 1.1 christos </tr> 54 1.1 christos <tr> 55 1.1 christos <th>From</th> 56 1.1 christos <th>To</th> 57 1.1 christos <th colspan="2">On</th> 58 1.1 christos <th>At</th> 59 1.1 christos <th>Action</th> 60 1.1 christos </tr> 61 1.1 christos <tr align="center"> 62 1.1 christos <td colspan="2">1920 only</td> 63 1.1 christos <td colspan="2">June 13<small><sup>th</sup></small></td> 64 1.1 christos <td rowspan="6">02:00 local</td> 65 1.1 christos <td>go to daylight saving time</td> 66 1.1 christos </tr> 67 1.1 christos <tr align="center"> 68 1.1 christos <td>1920</td> 69 1.1 christos <td>1921</td> 70 1.1 christos <td rowspan="5">last Sunday</td> 71 1.1 christos <td>in October</td> 72 1.1 christos <td>return to standard time</td> 73 1.1 christos </tr> 74 1.1 christos <tr align="center"> 75 1.1 christos <td colspan="2">1921 only</td> 76 1.1 christos <td>in March</td> 77 1.1 christos <td rowspan="2">go to daylight saving time</td> 78 1.1 christos </tr> 79 1.1 christos <tr align="center"> 80 1.1 christos <td rowspan="2">1922</td> 81 1.1 christos <td>1966</td> 82 1.1 christos <td>in April</td> 83 1.1 christos </tr> 84 1.1 christos <tr align="center"> 85 1.1 christos <td>1954</td> 86 1.1 christos <td>in September</td> 87 1.1 christos <td rowspan="2">return to standard time</td> 88 1.1 christos </tr> 89 1.1 christos <tr align="center"> 90 1.1 christos <td>1955</td> 91 1.1 christos <td>1966</td> 92 1.1 christos <td>in October</td> 93 1.1 christos </tr> 94 1.1 christos </table> 95 1.1 christos 96 1.1 christos <p>We’ll basically just ignore the <code>TYPE</code> column. 97 1.1 christos In the 2007j release, the most recent as of this writing, the 98 1.1 christos <code>TYPE</code> column never contains anything but a hyphen, 99 1.1 christos a kind of null value. (From the description in <code>zic.8.txt</code>, 100 1.1 christos this appears to be a mechanism for removing years from a set 101 1.1 christos in some localizable way. It’s used in the file, <code>pacificnew</code>, 102 1.1 christos to determine whether a given year will have a US presidential election; 103 1.1 christos but everything related to that use is commented out.) 104 1.1 christos 105 1.1 christos <p>The <code>SAVE</code> column contains the wall clock offset from 106 1.1 christos local standard time. 107 1.1 christos This is usually either zero for standard time or one hour for daylight 108 1.1 christos saving time; but there’s no reason, in principle, why it can’t 109 1.1 christos take on other values. 110 1.1 christos 111 1.1 christos <p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 112 1.1 christos column can contain a variable 113 1.1 christos part of the usual abbreviation of the time zone’s name, or it can just 114 1.1 christos be a hyphen if there’s no variable part. For example, the abbreviation 115 1.1 christos used in the central time zone will be either “CST” or 116 1.1 christos “CDT”. The variable part is ‘S’ or ‘D’; 117 1.1 christos and, sure enough, that’s just what we find in 118 1.1 christos the <code>LETTER</code> column 119 1.1 christos in the <code>Chicago</code> rules. More about this when we talk about 120 1.1 christos “Zone” lines. 121 1.1 christos 122 1.1 christos <p>One important thing to notice is that “Rule” lines 123 1.1 christos want at once to be both <i>transitions</i> and <i>steady states</i>: 124 1.1 christos <ul> 125 1.1 christos <li>On the one hand, they represent transitions between standard and 126 1.1 christos daylight saving time; and any number of Rule lines can be in effect 127 1.1 christos during a given period (which will always be a non-empty set of 128 1.1 christos contiguous calendar years).</li> 129 1.1 christos <li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 130 1.1 christos columns contain state that exists between transitions. More about this 131 1.1 christos when we talk about the US rules.</li> 132 1.1 christos </ul> 133 1.1 christos 134 1.1 christos <p>In the example above, the transition to daylight saving time 135 1.1 christos happened on the 13<small><sup>th</sup></small> of June in 1920, and on 136 1.1 christos the last Sunday in March in 1921; but the return to standard time 137 1.1 christos happened on the last Sunday in October in both of those 138 1.1 christos years. Similarly, the rule for changing to daylight saving time was 139 1.1 christos the same from 1922 to 1966; but the rule for returning to standard 140 1.1 christos time changed in 1955. Got it?</p> 141 1.1 christos 142 1.1 christos <p>OK, now for the somewhat more interesting “US” rules:</p> 143 1.1 christos 144 1.1 christos <table border="1"> 145 1.1 christos <tr> 146 1.1 christos <th colspan="6">From the Source File</th> 147 1.1 christos </tr> 148 1.1 christos <tr> 149 1.1 christos <td colspan="6" align="center"><table><tr><td> 150 1.1 christos <pre> 151 1.1 christos #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 152 1.1 christos Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 153 1.1 christos Rule US 1918 1919 - Oct lastSun 2:00 0 S 154 1.1 christos Rule US 1942 only - Feb 9 2:00 1:00 W # War 155 1.1 christos Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 156 1.1 christos Rule US 1945 only - Sep 30 2:00 0 S 157 1.1 christos Rule US 1967 2006 - Oct lastSun 2:00 0 S 158 1.1 christos Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 159 1.1 christos Rule US 1974 only - Jan 6 2:00 1:00 D 160 1.1 christos Rule US 1975 only - Feb 23 2:00 1:00 D 161 1.1 christos Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 162 1.1 christos Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 163 1.1 christos Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 164 1.1 christos Rule US 2007 max - Nov Sun>=1 2:00 0 S 165 1.1 christos </pre> 166 1.1 christos </td></tr></table></td> 167 1.1 christos </tr> 168 1.1 christos <tr> 169 1.1 christos <th colspan="6">Reformatted a Bit</th> 170 1.1 christos </tr> 171 1.1 christos <tr> 172 1.1 christos <th>From</th> 173 1.1 christos <th>To</th> 174 1.1 christos <th colspan="2">On</th> 175 1.1 christos <th>At</th> 176 1.1 christos <th>Action</th> 177 1.1 christos </tr> 178 1.1 christos <tr align="center"> 179 1.1 christos <td rowspan="2">1918</td> 180 1.1 christos <td rowspan="2">1919</td> 181 1.1 christos <td rowspan="2">last Sunday</td> 182 1.1 christos <td>in March</td> 183 1.1 christos <td rowspan="3">02:00 local</td> 184 1.1 christos <td>go to daylight saving time</td> 185 1.1 christos </tr> 186 1.1 christos <tr align="center"> 187 1.1 christos <td>in October</td> 188 1.1 christos <td>return to standard time</td> 189 1.1 christos </tr> 190 1.1 christos <tr align="center"> 191 1.1 christos <td colspan="2">1942 only</td> 192 1.1 christos <td colspan="2">February 9<small><sup>th</sup></small></td> 193 1.1 christos <td>go to “war time”</td> 194 1.1 christos </tr> 195 1.1 christos <tr align="center"> 196 1.1 christos <td colspan="2" rowspan="2">1945 only</td> 197 1.1 christos <td colspan="2">August 14<small><sup>th</sup></small></td> 198 1.1 christos <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 199 1.1 christos <td> 200 1.1 christos rename “war time” to “peace<br>time;” 201 1.1 christos clocks don’t change 202 1.1 christos </td> 203 1.1 christos </tr> 204 1.1 christos <tr align="center"> 205 1.1 christos <td colspan="2">September 30<small><sup>th</sup></small></td> 206 1.1 christos <td rowspan="9">02:00 local</td> 207 1.1 christos <td rowspan="2">return to standard time</td> 208 1.1 christos </tr> 209 1.1 christos <tr align="center"> 210 1.1 christos <td rowspan="2">1967</td> 211 1.1 christos <td>2006</td> 212 1.1 christos <td rowspan="2">last Sunday</td> 213 1.1 christos <td>in October</td> 214 1.1 christos </tr> 215 1.1 christos <tr align="center"> 216 1.1 christos <td>1973</td> 217 1.1 christos <td>in April</td> 218 1.1 christos <td rowspan="6">go to daylight saving time</td> 219 1.1 christos </tr> 220 1.1 christos <tr align="center"> 221 1.1 christos <td colspan="2">1974 only</td> 222 1.1 christos <td colspan="2">January 6<small><sup>th</sup></small></td> 223 1.1 christos </tr> 224 1.1 christos <tr align="center"> 225 1.1 christos <td colspan="2">1975 only</td> 226 1.1 christos <td colspan="2">February 23<small><sup>rd</sup></small></td> 227 1.1 christos </tr> 228 1.1 christos <tr align="center"> 229 1.1 christos <td>1976</td> 230 1.1 christos <td>1986</td> 231 1.1 christos <td>last Sunday</td> 232 1.1 christos <td rowspan="2">in April</td> 233 1.1 christos </tr> 234 1.1 christos <tr align="center"> 235 1.1 christos <td>1987</td> 236 1.1 christos <td>2006</td> 237 1.1 christos <td>first Sunday</td> 238 1.1 christos </tr> 239 1.1 christos <tr align="center"> 240 1.1 christos <td rowspan="2">2007</td> 241 1.1 christos <td rowspan="2">present</td> 242 1.1 christos <td colspan="2">second Sunday in March</td> 243 1.1 christos </tr> 244 1.1 christos <tr align="center"> 245 1.1 christos <td colspan="2">first Sunday in November</td> 246 1.1 christos <td>return to standard time</td> 247 1.1 christos </tr> 248 1.1 christos </table> 249 1.1 christos 250 1.1 christos <p>There are two interesting things to note here.</p> 251 1.1 christos 252 1.1 christos <p>First, the time that something happens (in the <code>AT</code> 253 1.1 christos column) is not necessarily the local wall clock time. The time can be 254 1.1 christos suffixed with ‘s’ (for “standard”) to mean 255 1.1 christos local standard time (different from wall clock time when observing 256 1.1 christos daylight saving time); or it can be suffixed with ‘g’, 257 1.1 christos ‘u’, or ‘z’, all three of which mean the 258 1.1 christos standard time at the 259 1.2 christos <a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 260 1.1 christos ‘g’ stands for “<a 261 1.1 christos href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 262 1.1 christos ‘u’ stands for “<a 263 1.1 christos href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 264 1.1 christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 265 1.1 christos (whichever was official at the time); ‘z’ stands for the 266 1.1 christos <a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 267 1.1 christos Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 268 1.1 christos The time can also be suffixed with ‘w’ meaning “wall 269 1.1 christos clock time;” but it usually isn’t because that’s the 270 1.1 christos default.</p> 271 1.1 christos 272 1.1 christos <p>Second, the day in the <code>ON</code> column, in addition to 273 1.1 christos “<code>lastSun</code>” or a particular day of the month, 274 1.1 christos can have the form, “<code>Sun>=</code><i>x</i>” or 275 1.1 christos “<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 276 1.1 christos of the month. For example, “<code>Sun>=8</code>” means 277 1.1 christos “the first Sunday on or after the eighth of the month,” in 278 1.1 christos other words, the second Sunday of the month. Furthermore, although 279 1.1 christos there are no examples above, the weekday needn’t be 280 1.1 christos “<code>Sun</code>” in either form, but can be the usual 281 1.1 christos three-character English abbreviation for any day of the week.</p> 282 1.1 christos 283 1.1 christos <p>And the US rules give us more examples of a couple of things 284 1.1 christos already mentioned:</p> 285 1.1 christos 286 1.1 christos <ul> 287 1.1 christos <li>The rules for changing to and from daylight saving time are 288 1.1 christos actually <i>different sets</i> of rules; and the two sets can change 289 1.1 christos independently. Consider, for example, that the rule for the return to 290 1.1 christos standard time stayed the same from 1967 to 2006; but the rule for the 291 1.1 christos transition to daylight saving time changed several times in the same 292 1.1 christos period. There can also be periods, 1946 to 1966 for example, when no 293 1.1 christos rule from this group is in effect, and so either no transition 294 1.1 christos happened in those years, or some other rule is in effect (perhaps a 295 1.1 christos state or other more local rule).</li> 296 1.1 christos 297 1.1 christos <li>The <code>SAVE</code> and <code>LETTER</code> columns 298 1.1 christos contain <i>steady state</i>, not transitions. Consider, for example, 299 1.1 christos the transition from “war time” to “peace time” 300 1.1 christos that happened on August 14, 1945. The “1:00” in 301 1.1 christos the <code>SAVE</code> column is <i>not</i> an instruction to advance 302 1.1 christos the clock an hour. It means that clocks should <i>be</i> one hour 303 1.1 christos ahead of standard time, which they already are because of the previous 304 1.1 christos rule, so there should be no change.</li> 305 1.1 christos 306 1.1 christos </ul> 307 1.1 christos 308 1.1 christos <p>OK, now let’s look at a Zone record:</p> 309 1.1 christos 310 1.1 christos <table border="1"> 311 1.1 christos <tr> 312 1.1 christos <th colspan="5">From the Source File</th> 313 1.1 christos </tr> 314 1.1 christos <tr> 315 1.1 christos <td colspan="6" align="center"><table><tr><td> 316 1.1 christos <pre> 317 1.1 christos #Zone NAME GMTOFF RULES FORMAT [UNTIL] 318 1.1 christos Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 319 1.1 christos -6:00 US C%sT 1920 320 1.1 christos -6:00 Chicago C%sT 1936 Mar 1 2:00 321 1.1 christos -5:00 - EST 1936 Nov 15 2:00 322 1.1 christos -6:00 Chicago C%sT 1942 323 1.1 christos -6:00 US C%sT 1946 324 1.1 christos -6:00 Chicago C%sT 1967 325 1.1 christos -6:00 US C%sT 326 1.1 christos </pre> 327 1.1 christos </td></tr></table></td> 328 1.1 christos </tr> 329 1.1 christos <tr> 330 1.1 christos <th colspan="5">Columns Renamed</th> 331 1.1 christos </tr> 332 1.1 christos <tr> 333 1.1 christos <th rowspan="2">Standard Offset<br> 334 1.1 christos from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 335 1.1 christos Meridian</a></th> 336 1.1 christos <th rowspan="2">Daylight<br>Saving Time</th> 337 1.1 christos <th rowspan="2">Abbreviation(s)</th> 338 1.1 christos <th colspan="2">Ending at Local Time</th> 339 1.1 christos </tr> 340 1.1 christos <tr> 341 1.1 christos <th>Date</th> 342 1.1 christos <th>Time</th> 343 1.1 christos </tr> 344 1.1 christos <tr align="center"> 345 1.1 christos <td>−5:50:36</td> 346 1.1 christos <td>not observed</td> 347 1.1 christos <td>LMT</td> 348 1.1 christos <td>1883-11-18</td> 349 1.1 christos <td>12:09:24</td> 350 1.1 christos </tr> 351 1.1 christos <tr align="center"> 352 1.1 christos <td rowspan="2">−6:00:00</td> 353 1.1 christos <td>US rules</td> 354 1.1 christos <td rowspan="2">CST or CDT</td> 355 1.1 christos <td>1920-01-01</td> 356 1.1 christos <td>00:00:00</td> 357 1.1 christos </tr> 358 1.1 christos <tr align="center"> 359 1.1 christos <td>Chicago rules</td> 360 1.1 christos <td>1936-03-01</td> 361 1.1 christos <td rowspan="2">02:00:00</td> 362 1.1 christos </tr> 363 1.1 christos <tr align="center"> 364 1.1 christos <td>−5:00:00</td> 365 1.1 christos <td>not observed</td> 366 1.1 christos <td>EST</td> 367 1.1 christos <td>1936-11-15</td> 368 1.1 christos </tr> 369 1.1 christos <tr align="center"> 370 1.1 christos <td rowspan="4">−6:00:00</td> 371 1.1 christos <td>Chicago rules</td> 372 1.1 christos <td>CST or CDT</td> 373 1.1 christos <td>1942-01-01</td> 374 1.1 christos <td rowspan="3">00:00:00</td> 375 1.1 christos </tr> 376 1.1 christos <tr align="center"> 377 1.1 christos <td>US rules</td> 378 1.1 christos <td>CST, CWT or CPT</td> 379 1.1 christos <td>1946-01-01</td> 380 1.1 christos </tr> 381 1.1 christos <tr align="center"> 382 1.1 christos <td>Chicago rules</td> 383 1.1 christos <td rowspan="2">CST or CDT</td> 384 1.1 christos <td>1967-01-01</td> 385 1.1 christos </tr> 386 1.1 christos <tr align="center"> 387 1.1 christos <td>US rules</td> 388 1.1 christos <td colspan="2">—</td> 389 1.1 christos </tr> 390 1.1 christos </table> 391 1.1 christos 392 1.1 christos <p>There are a couple of interesting differences between Zones and Rules.</p> 393 1.1 christos 394 1.1 christos <p>First, and somewhat trivially, whereas Rules are considered to 395 1.1 christos contain one or more records, a Zone is considered to be a single 396 1.1 christos record with zero or more <i>continuation lines</i>. Thus, the keyword, 397 1.1 christos “<code>Zone</code>,” and the zone name are not 398 1.1 christos repeated. The last line is the one without anything in 399 1.1 christos the <code>[UNTIL]</code> column.</p> 400 1.1 christos 401 1.1 christos <p>Second, and more fundamentally, each line of a Zone represents a 402 1.1 christos steady state, not a transition between states. The state exists from 403 1.1 christos the date and time in the previous line’s <code>[UNTIL]</code> 404 1.1 christos column up to the date and time in the current 405 1.1 christos line’s <code>[UNTIL]</code> column. In other words, the date and 406 1.1 christos time in the <code>[UNTIL]</code> column is the instant that separates 407 1.1 christos this state from the next. Where that would be ambiguous because 408 1.1 christos we’re setting our clocks back, the <code>[UNTIL]</code> column 409 1.1 christos specifies the first occurrence of the instant. The state specified by 410 1.1 christos the last line, the one without anything in the <code>[UNTIL]</code> 411 1.1 christos column, continues to the present.</p> 412 1.1 christos 413 1.1 christos <p>The first line typically specifies the mean solar time observed 414 1.1 christos before the introduction of standard time. Since there’s no line before 415 1.1 christos that, it has no beginning. <code>8-) </code> For some places near the <a 416 1.1 christos href="https://en.wikipedia.org/wiki/International_Date_Line">International 417 1.1 christos Date Line</a>, the first <i>two</i> lines will show solar times 418 1.1 christos differing by 24 hours; this corresponds to a movement of the Date 419 1.1 christos Line. For example:</p> 420 1.1 christos 421 1.1 christos <pre> 422 1.1 christos #Zone NAME GMTOFF RULES FORMAT [UNTIL] 423 1.1 christos Zone America/Juneau 15:02:19 - LMT 1867 Oct 18 424 1.1 christos -8:57:41 - LMT ... 425 1.1 christos </pre> 426 1.1 christos 427 1.1 christos <p>When Alaska was purchased from Russia in 1867, the Date Line moved 428 1.1 christos from the Alaska/Canada border to the Bering Strait; and the time in 429 1.1 christos Alaska was then 24 hours earlier than it had 430 1.1 christos been. <code><aside></code>(6 October in the Julian calendar, 431 1.1 christos which Russia was still using then for religious reasons, was followed 432 1.1 christos by <i>a second instance of the same day with a different name</i>, 18 433 1.1 christos October in the Gregorian calendar. Isn’t civil time 434 1.1 christos wonderful? <code>8-)</code>)<code></aside></code></p> 435 1.1 christos 436 1.1 christos <p>The abbreviation, “LMT” stands for “local mean 437 1.1 christos time”, which is an invention of 438 1.1 christos the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 439 1.1 christos database</a> and was probably never actually used during the 440 1.1 christos period. Furthermore, the value is almost certainly wrong except in the 441 1.1 christos archetypal place after which the zone is named. (The tz database 442 1.1 christos usually doesn’t provide a separate Zone record for places where 443 1.1 christos nothing significant happened after 1970.)</p> 444 1.1 christos 445 1.1 christos <p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 446 1.1 christos <ul> 447 1.1 christos <li>A hyphen, a kind of null value, means that we have not set our 448 1.1 christos clocks ahead of standard time.</li> 449 1.1 christos 450 1.1 christos <li>An amount of time (usually but not necessarily “1:00” 451 1.1 christos meaning one hour) means that we have set our clocks ahead by that 452 1.1 christos amount.</li> 453 1.1 christos 454 1.1 christos <li>Some alphabetic string means that we <i>might have</i> set our 455 1.1 christos clocks ahead; and we need to check the rule the name of which is the 456 1.1 christos given alphabetic string.</li> 457 1.1 christos </ul> 458 1.1 christos 459 1.1 christos <p>An example of a specific amount of time is:</p> 460 1.1 christos <pre> 461 1.1 christos #Zone NAME GMTOFF RULES FORMAT [UNTIL] 462 1.6 christos Zone Pacific/Honolulu ... 1933 Apr 30 2:00 463 1.6 christos -10:30 1:00 HDT 1933 May 21 12:00 464 1.1 christos ... 465 1.1 christos </pre> 466 1.1 christos 467 1.1 christos <p>Hawaii tried daylight saving time for three weeks in 1933 and 468 1.1 christos decided they didn’t like it. <code>8-) </code>Note that 469 1.1 christos the <code>GMTOFF</code> column always contains the standard time 470 1.1 christos offset, so the wall clock time during this period was GMT − 471 1.1 christos 10:30 + 1:00 = GMT − 9:30.</p> 472 1.1 christos 473 1.1 christos <p>The <code>FORMAT</code> column specifies the usual abbreviation of 474 1.1 christos the time zone name. It can have one of three forms:</p> 475 1.1 christos <ul> 476 1.1 christos 477 1.1 christos <li>a string of three or more characters that are either ASCII alphanumerics, 478 1.1 christos “<code>+</code>”, or “<code>-</code>”, 479 1.1 christos in which case that’s the abbreviation</li> 480 1.1 christos 481 1.1 christos <li>a pair of strings separated by a slash 482 1.1 christos (‘<code>/</code>’), in which case the first string is the 483 1.1 christos abbreviation for the standard time name and the second string is the 484 1.1 christos abbreviation for the daylight saving time name</li> 485 1.1 christos 486 1.1 christos <li>a string containing “<code>%s</code>,” in which case 487 1.1 christos the “<code>%s</code>” will be replaced by the text in the 488 1.1 christos appropriate Rule’s <code>LETTER</code> column</li> 489 1.1 christos </ul> 490 1.1 christos 491 1.1 christos <p>The last two make sense only if there’s a named rule in effect.</p> 492 1.1 christos 493 1.1 christos <p>An example of a slash is:</p> 494 1.1 christos <pre> 495 1.1 christos #Zone NAME GMTOFF RULES FORMAT [UNTIL] 496 1.1 christos Zone Europe/London ... 1996 497 1.1 christos 0:00 EU GMT/BST 498 1.1 christos </pre> 499 1.1 christos 500 1.1 christos <p>The current time in the UK is called either Greenwich mean time or 501 1.1 christos British summer time.</p> 502 1.1 christos 503 1.1 christos <p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 504 1.1 christos happens when switching to a named rule. To what values should 505 1.1 christos the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 506 1.1 christos 507 1.1 christos <ul> 508 1.1 christos <li>If at least one transition has happened, use 509 1.1 christos the <code>SAVE</code> and <code>LETTER</code> data from the most 510 1.1 christos recent.</li> 511 1.1 christos 512 1.1 christos <li>If switching to a named rule before any transition has happened, 513 1.1 christos assume standard time (<code>SAVE</code> zero), and use 514 1.1 christos the <code>LETTER</code> data from the earliest transition with 515 1.1 christos a <code>SAVE</code> of zero. 516 1.1 christos 517 1.1 christos </ul> 518 1.1 christos 519 1.1 christos <p>And three last things about the <code>FORMAT</code> column:</p> 520 1.1 christos <ul> 521 1.1 christos 522 1.1 christos <li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 523 1.5 christos database</a> gives abbreviations for time zones in <i>popular 524 1.1 christos usage</i>, which is not necessarily “correct” by law. For 525 1.1 christos example, the last line in 526 1.1 christos <code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 527 1.1 christos “HST” for “Hawaii standard time” even though the 528 1.3 christos <a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a> 529 1.1 christos name for that time zone is “Hawaii-Aleutian standard time.” 530 1.1 christos This author has read that there are also some places in Australia where 531 1.1 christos popular time zone names differ from the legal ones. 532 1.1 christos 533 1.1 christos <li>No attempt is made to <a 534 1.1 christos href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 535 1.1 christos the abbreviations. They are intended to be the values returned through the 536 1.1 christos <code>"%Z"</code> format specifier to 537 1.1 christos <a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 538 1.1 christos <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 539 1.1 christos function in the 540 1.3 christos <a href="http://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 541 1.1 christos 542 1.1 christos <li>If there is no generally-accepted abbreviation for a time zone, 543 1.1 christos a numeric offset is used instead, e.g., <code>+07</code> for 7 hours 544 1.1 christos ahead of Greenwich. By convention, <code>-00</code> is used in a 545 1.1 christos zone while uninhabited, where the offset is zero but in some sense 546 1.1 christos the true offset is undefined. 547 1.1 christos </ul> 548 1.1 christos 549 1.1 christos <p>As a final example, here’s the complete history for Hawaii:</p> 550 1.1 christos 551 1.1 christos <table border="1"> 552 1.1 christos <tr> 553 1.1 christos <th colspan="6">Relevant Excerpts from the US Rules</th> 554 1.1 christos </tr> 555 1.1 christos <tr> 556 1.1 christos <td colspan="6" align="center"><table><tr><td> 557 1.1 christos <pre> 558 1.1 christos #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 559 1.1 christos Rule US 1918 1919 - Oct lastSun 2:00 0 S 560 1.1 christos Rule US 1942 only - Feb 9 2:00 1:00 W # War 561 1.1 christos Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 562 1.6 christos Rule US 1945 only - Sep lastSun 2:00 0 S 563 1.1 christos </pre> 564 1.1 christos </td></tr></table></td> 565 1.1 christos </tr> 566 1.1 christos <tr> 567 1.1 christos <th colspan="6">The Zone Record</th> 568 1.1 christos </tr> 569 1.1 christos <tr> 570 1.1 christos <td colspan="6" align="center"><table><tr><td> 571 1.1 christos <pre> 572 1.1 christos #Zone NAME GMTOFF RULES FORMAT [UNTIL] 573 1.6 christos Zone Pacific/Honolulu -10:31:26 - LMT 1896 Jan 13 12:00 574 1.1 christos -10:30 - HST 1933 Apr 30 2:00 575 1.1 christos -10:30 1:00 HDT 1933 May 21 2:00 576 1.1 christos -10:30 US H%sT 1947 Jun 8 2:00 577 1.1 christos -10:00 - HST 578 1.1 christos </pre> 579 1.1 christos </td></tr></table></td> 580 1.1 christos </tr> 581 1.1 christos <tr> 582 1.1 christos <th colspan="6">What We Infer</th> 583 1.1 christos </tr> 584 1.1 christos <tr> 585 1.1 christos <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 586 1.1 christos <th rowspan="2">Adjust<br>Clocks</th> 587 1.1 christos <th colspan="2">Time Zone</th> 588 1.1 christos <th colspan="2">Ending at Local Time</th> 589 1.1 christos </tr> 590 1.1 christos <tr> 591 1.1 christos <th>Abbrv.</th> 592 1.1 christos <th>Name</th> 593 1.1 christos <th>Date</th> 594 1.1 christos <th>Time</th> 595 1.1 christos </tr> 596 1.1 christos <tr align="center"> 597 1.1 christos <td>−10:31:26</td> 598 1.1 christos <td>—</td> 599 1.1 christos <td>LMT</td> 600 1.1 christos <td>local mean time</td> 601 1.6 christos <td>1896-01-13</td> 602 1.1 christos <td>12:00</td> 603 1.1 christos </tr> 604 1.1 christos <tr align="center"> 605 1.1 christos <td>−10:30</td> 606 1.1 christos <td>+0:01:26</td> 607 1.1 christos <td>HST</td> 608 1.1 christos <td>Hawaii standard time</td> 609 1.1 christos <td>1933-04-30</td> 610 1.6 christos <td>02:00</td> 611 1.1 christos </tr> 612 1.1 christos <tr align="center"> 613 1.1 christos <td>−9:30</td> 614 1.1 christos <td>+1:00</td> 615 1.1 christos <td>HDT</td> 616 1.1 christos <td>Hawaii daylight time</td> 617 1.1 christos <td>1933-05-21</td> 618 1.6 christos <td>12:00</td> 619 1.1 christos </tr> 620 1.1 christos <tr align="center"> 621 1.1 christos <td>−10:30¹</td> 622 1.1 christos <td>−1:00¹</td> 623 1.1 christos <td>HST¹</td> 624 1.1 christos <td>Hawaii standard time</td> 625 1.1 christos <td>1942-02-09</td> 626 1.6 christos <td>02:00</td> 627 1.1 christos </tr> 628 1.1 christos <tr align="center"> 629 1.1 christos <td rowspan="2">−9:30</td> 630 1.1 christos <td>+1:00</td> 631 1.1 christos <td>HWT</td> 632 1.1 christos <td>Hawaii war time</td> 633 1.1 christos <td>1945-08-14</td> 634 1.1 christos <td>13:30²</td> 635 1.1 christos </tr> 636 1.1 christos <tr align="center"> 637 1.1 christos <td>0</td> 638 1.1 christos <td>HPT</td> 639 1.1 christos <td>Hawaii peace time</td> 640 1.1 christos <td>1945-09-30</td> 641 1.1 christos <td rowspan="2">02:00</td> 642 1.1 christos </tr> 643 1.1 christos <tr align="center"> 644 1.1 christos <td>−10:30</td> 645 1.1 christos <td>−1:00</td> 646 1.1 christos <td rowspan="2">HST</td> 647 1.1 christos <td rowspan="2">Hawaii standard time</td> 648 1.1 christos <td>1947-06-08</td> 649 1.1 christos </tr> 650 1.1 christos <tr align="center"> 651 1.1 christos <td>−10:00³</td> 652 1.1 christos <td>+0:30³</td> 653 1.1 christos <td colspan="2">—</td> 654 1.1 christos </tr> 655 1.1 christos <tr> 656 1.1 christos <td colspan="6"> 657 1.1 christos ¹Switching to US rules…most recent transition (in 1919) was to standard time 658 1.1 christos </td> 659 1.1 christos </tr> 660 1.1 christos <tr> 661 1.1 christos <td colspan="6"> 662 1.1 christos ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 663 1.1 christos + (−9:30) = 13:30 local 664 1.1 christos </td> 665 1.1 christos </tr> 666 1.1 christos <tr> 667 1.1 christos <td colspan="6"> 668 1.1 christos ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 669 1.1 christos the civil time in Hawaii has been 670 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> 671 1.1 christos − 10:00 year-round. 672 1.1 christos </td> 673 1.1 christos </tr> 674 1.1 christos </table> 675 1.1 christos 676 1.1 christos <p>There will be a short quiz later. <code>8-)</code></p> 677 1.1 christos 678 1.1 christos <hr> 679 1.1 christos <address> 680 1.1 christos This web page is in the public domain, so clarified as of 681 1.1 christos 2015-10-20 by Bill Seymour. 682 1.1 christos <br> 683 1.1 christos All suggestions and corrections will be welcome; all flames will be amusing. 684 1.1 christos Mail to was at pobox dot com. 685 1.1 christos </address> 686 1.1 christos </body> 687 1.1 christos </html> 688
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