tz-how-to.html revision 1.3
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 1.3 christos </head> 9 1.1 christos <body> 10 1.1 christos <h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 11 1.1 christos Database</a> Source Files</h2> 12 1.1 christos <h3>by Bill Seymour</h3> 13 1.1 christos <p>This page uses the <code>America/Chicago</code> and 14 1.1 christos <code>Pacific/Honolulu</code> zones as examples of how to infer 15 1.1 christos times of day from the <a href="tz-link.htm">tz database</a> 16 1.1 christos source files. It might be helpful, but not absolutely necessary, 17 1.1 christos for the reader to have already downloaded the 18 1.2 christos latest release of the database and become familiar with the basic layout 19 1.1 christos of the data files. The format is explained in the “man 20 1.1 christos page” for the zic compiler, <code>zic.8.txt</code>, in 21 1.1 christos the <code>code</code> subdirectory.</p> 22 1.1 christos 23 1.1 christos <p>We’ll begin by talking about the rules for changing between standard 24 1.1 christos and daylight saving time since we’ll need that information when we talk 25 1.1 christos about the zones.</p> 26 1.1 christos 27 1.1 christos <p>First, let’s consider the special daylight saving time rules 28 1.1 christos for Chicago (from the <code>northamerica</code> file in 29 1.1 christos the <code>data</code> subdirectory):</p> 30 1.1 christos 31 1.1 christos <table border="1"> 32 1.1 christos <tr> 33 1.1 christos <th colspan="6">From the Source File</th> 34 1.1 christos </tr> 35 1.1 christos <tr> 36 1.1 christos <td colspan="6" align="center"><table><tr><td> 37 1.1 christos <pre> 38 1.1 christos #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER 39 1.1 christos Rule Chicago 1920 only - Jun 13 2:00 1:00 D 40 1.1 christos Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 41 1.1 christos Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 42 1.1 christos Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 43 1.1 christos Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 44 1.1 christos Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S 45 1.1 christos </pre> 46 1.1 christos </td></tr></table></td> 47 1.1 christos </tr> 48 1.1 christos <tr> 49 1.1 christos <th colspan="6">Reformatted a Bit</th> 50 1.1 christos </tr> 51 1.1 christos <tr> 52 1.1 christos <th>From</th> 53 1.1 christos <th>To</th> 54 1.1 christos <th colspan="2">On</th> 55 1.1 christos <th>At</th> 56 1.1 christos <th>Action</th> 57 1.1 christos </tr> 58 1.1 christos <tr align="center"> 59 1.1 christos <td colspan="2">1920 only</td> 60 1.1 christos <td colspan="2">June 13<small><sup>th</sup></small></td> 61 1.1 christos <td rowspan="6">02:00 local</td> 62 1.1 christos <td>go to daylight saving time</td> 63 1.1 christos </tr> 64 1.1 christos <tr align="center"> 65 1.1 christos <td>1920</td> 66 1.1 christos <td>1921</td> 67 1.1 christos <td rowspan="5">last Sunday</td> 68 1.1 christos <td>in October</td> 69 1.1 christos <td>return to standard time</td> 70 1.1 christos </tr> 71 1.1 christos <tr align="center"> 72 1.1 christos <td colspan="2">1921 only</td> 73 1.1 christos <td>in March</td> 74 1.1 christos <td rowspan="2">go to daylight saving time</td> 75 1.1 christos </tr> 76 1.1 christos <tr align="center"> 77 1.1 christos <td rowspan="2">1922</td> 78 1.1 christos <td>1966</td> 79 1.1 christos <td>in April</td> 80 1.1 christos </tr> 81 1.1 christos <tr align="center"> 82 1.1 christos <td>1954</td> 83 1.1 christos <td>in September</td> 84 1.1 christos <td rowspan="2">return to standard time</td> 85 1.1 christos </tr> 86 1.1 christos <tr align="center"> 87 1.1 christos <td>1955</td> 88 1.1 christos <td>1966</td> 89 1.1 christos <td>in October</td> 90 1.1 christos </tr> 91 1.1 christos </table> 92 1.1 christos 93 1.1 christos <p>We’ll basically just ignore the <code>TYPE</code> column. 94 1.1 christos In the 2007j release, the most recent as of this writing, the 95 1.1 christos <code>TYPE</code> column never contains anything but a hyphen, 96 1.1 christos a kind of null value. (From the description in <code>zic.8.txt</code>, 97 1.1 christos this appears to be a mechanism for removing years from a set 98 1.1 christos in some localizable way. It’s used in the file, <code>pacificnew</code>, 99 1.1 christos to determine whether a given year will have a US presidential election; 100 1.1 christos but everything related to that use is commented out.) 101 1.1 christos 102 1.1 christos <p>The <code>SAVE</code> column contains the wall clock offset from 103 1.1 christos local standard time. 104 1.1 christos This is usually either zero for standard time or one hour for daylight 105 1.1 christos saving time; but there’s no reason, in principle, why it can’t 106 1.1 christos take on other values. 107 1.1 christos 108 1.1 christos <p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 109 1.1 christos column can contain a variable 110 1.1 christos part of the usual abbreviation of the time zone’s name, or it can just 111 1.1 christos be a hyphen if there’s no variable part. For example, the abbreviation 112 1.1 christos used in the central time zone will be either “CST” or 113 1.1 christos “CDT”. The variable part is ‘S’ or ‘D’; 114 1.1 christos and, sure enough, that’s just what we find in 115 1.1 christos the <code>LETTER</code> column 116 1.1 christos in the <code>Chicago</code> rules. More about this when we talk about 117 1.1 christos “Zone” lines. 118 1.1 christos 119 1.1 christos <p>One important thing to notice is that “Rule” lines 120 1.1 christos want at once to be both <i>transitions</i> and <i>steady states</i>: 121 1.1 christos <ul> 122 1.1 christos <li>On the one hand, they represent transitions between standard and 123 1.1 christos daylight saving time; and any number of Rule lines can be in effect 124 1.1 christos during a given period (which will always be a non-empty set of 125 1.1 christos contiguous calendar years).</li> 126 1.1 christos <li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 127 1.1 christos columns contain state that exists between transitions. More about this 128 1.1 christos when we talk about the US rules.</li> 129 1.1 christos </ul> 130 1.1 christos 131 1.1 christos <p>In the example above, the transition to daylight saving time 132 1.1 christos happened on the 13<small><sup>th</sup></small> of June in 1920, and on 133 1.1 christos the last Sunday in March in 1921; but the return to standard time 134 1.1 christos happened on the last Sunday in October in both of those 135 1.1 christos years. Similarly, the rule for changing to daylight saving time was 136 1.1 christos the same from 1922 to 1966; but the rule for returning to standard 137 1.1 christos time changed in 1955. Got it?</p> 138 1.1 christos 139 1.1 christos <p>OK, now for the somewhat more interesting “US” rules:</p> 140 1.1 christos 141 1.1 christos <table border="1"> 142 1.1 christos <tr> 143 1.1 christos <th colspan="6">From the Source File</th> 144 1.1 christos </tr> 145 1.1 christos <tr> 146 1.1 christos <td colspan="6" align="center"><table><tr><td> 147 1.1 christos <pre> 148 1.1 christos #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 149 1.1 christos Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 150 1.1 christos Rule US 1918 1919 - Oct lastSun 2:00 0 S 151 1.1 christos Rule US 1942 only - Feb 9 2:00 1:00 W # War 152 1.1 christos Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 153 1.1 christos Rule US 1945 only - Sep 30 2:00 0 S 154 1.1 christos Rule US 1967 2006 - Oct lastSun 2:00 0 S 155 1.1 christos Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 156 1.1 christos Rule US 1974 only - Jan 6 2:00 1:00 D 157 1.1 christos Rule US 1975 only - Feb 23 2:00 1:00 D 158 1.1 christos Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 159 1.1 christos Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 160 1.1 christos Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 161 1.1 christos Rule US 2007 max - Nov Sun>=1 2:00 0 S 162 1.1 christos </pre> 163 1.1 christos </td></tr></table></td> 164 1.1 christos </tr> 165 1.1 christos <tr> 166 1.1 christos <th colspan="6">Reformatted a Bit</th> 167 1.1 christos </tr> 168 1.1 christos <tr> 169 1.1 christos <th>From</th> 170 1.1 christos <th>To</th> 171 1.1 christos <th colspan="2">On</th> 172 1.1 christos <th>At</th> 173 1.1 christos <th>Action</th> 174 1.1 christos </tr> 175 1.1 christos <tr align="center"> 176 1.1 christos <td rowspan="2">1918</td> 177 1.1 christos <td rowspan="2">1919</td> 178 1.1 christos <td rowspan="2">last Sunday</td> 179 1.1 christos <td>in March</td> 180 1.1 christos <td rowspan="3">02:00 local</td> 181 1.1 christos <td>go to daylight saving time</td> 182 1.1 christos </tr> 183 1.1 christos <tr align="center"> 184 1.1 christos <td>in October</td> 185 1.1 christos <td>return to standard time</td> 186 1.1 christos </tr> 187 1.1 christos <tr align="center"> 188 1.1 christos <td colspan="2">1942 only</td> 189 1.1 christos <td colspan="2">February 9<small><sup>th</sup></small></td> 190 1.1 christos <td>go to “war time”</td> 191 1.1 christos </tr> 192 1.1 christos <tr align="center"> 193 1.1 christos <td colspan="2" rowspan="2">1945 only</td> 194 1.1 christos <td colspan="2">August 14<small><sup>th</sup></small></td> 195 1.1 christos <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 196 1.1 christos <td> 197 1.1 christos rename “war time” to “peace<br>time;” 198 1.1 christos clocks don’t change 199 1.1 christos </td> 200 1.1 christos </tr> 201 1.1 christos <tr align="center"> 202 1.1 christos <td colspan="2">September 30<small><sup>th</sup></small></td> 203 1.1 christos <td rowspan="9">02:00 local</td> 204 1.1 christos <td rowspan="2">return to standard time</td> 205 1.1 christos </tr> 206 1.1 christos <tr align="center"> 207 1.1 christos <td rowspan="2">1967</td> 208 1.1 christos <td>2006</td> 209 1.1 christos <td rowspan="2">last Sunday</td> 210 1.1 christos <td>in October</td> 211 1.1 christos </tr> 212 1.1 christos <tr align="center"> 213 1.1 christos <td>1973</td> 214 1.1 christos <td>in April</td> 215 1.1 christos <td rowspan="6">go to daylight saving time</td> 216 1.1 christos </tr> 217 1.1 christos <tr align="center"> 218 1.1 christos <td colspan="2">1974 only</td> 219 1.1 christos <td colspan="2">January 6<small><sup>th</sup></small></td> 220 1.1 christos </tr> 221 1.1 christos <tr align="center"> 222 1.1 christos <td colspan="2">1975 only</td> 223 1.1 christos <td colspan="2">February 23<small><sup>rd</sup></small></td> 224 1.1 christos </tr> 225 1.1 christos <tr align="center"> 226 1.1 christos <td>1976</td> 227 1.1 christos <td>1986</td> 228 1.1 christos <td>last Sunday</td> 229 1.1 christos <td rowspan="2">in April</td> 230 1.1 christos </tr> 231 1.1 christos <tr align="center"> 232 1.1 christos <td>1987</td> 233 1.1 christos <td>2006</td> 234 1.1 christos <td>first Sunday</td> 235 1.1 christos </tr> 236 1.1 christos <tr align="center"> 237 1.1 christos <td rowspan="2">2007</td> 238 1.1 christos <td rowspan="2">present</td> 239 1.1 christos <td colspan="2">second Sunday in March</td> 240 1.1 christos </tr> 241 1.1 christos <tr align="center"> 242 1.1 christos <td colspan="2">first Sunday in November</td> 243 1.1 christos <td>return to standard time</td> 244 1.1 christos </tr> 245 1.1 christos </table> 246 1.1 christos 247 1.1 christos <p>There are two interesting things to note here.</p> 248 1.1 christos 249 1.1 christos <p>First, the time that something happens (in the <code>AT</code> 250 1.1 christos column) is not necessarily the local wall clock time. The time can be 251 1.1 christos suffixed with ‘s’ (for “standard”) to mean 252 1.1 christos local standard time (different from wall clock time when observing 253 1.1 christos daylight saving time); or it can be suffixed with ‘g’, 254 1.1 christos ‘u’, or ‘z’, all three of which mean the 255 1.1 christos standard time at the 256 1.2 christos <a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 257 1.1 christos ‘g’ stands for “<a 258 1.1 christos href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 259 1.1 christos ‘u’ stands for “<a 260 1.1 christos href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 261 1.1 christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 262 1.1 christos (whichever was official at the time); ‘z’ stands for the 263 1.1 christos <a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 264 1.1 christos Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 265 1.1 christos The time can also be suffixed with ‘w’ meaning “wall 266 1.1 christos clock time;” but it usually isn’t because that’s the 267 1.1 christos default.</p> 268 1.1 christos 269 1.1 christos <p>Second, the day in the <code>ON</code> column, in addition to 270 1.1 christos “<code>lastSun</code>” or a particular day of the month, 271 1.1 christos can have the form, “<code>Sun>=</code><i>x</i>” or 272 1.1 christos “<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 273 1.1 christos of the month. For example, “<code>Sun>=8</code>” means 274 1.1 christos “the first Sunday on or after the eighth of the month,” in 275 1.1 christos other words, the second Sunday of the month. Furthermore, although 276 1.1 christos there are no examples above, the weekday needn’t be 277 1.1 christos “<code>Sun</code>” in either form, but can be the usual 278 1.1 christos three-character English abbreviation for any day of the week.</p> 279 1.1 christos 280 1.1 christos <p>And the US rules give us more examples of a couple of things 281 1.1 christos already mentioned:</p> 282 1.1 christos 283 1.1 christos <ul> 284 1.1 christos <li>The rules for changing to and from daylight saving time are 285 1.1 christos actually <i>different sets</i> of rules; and the two sets can change 286 1.1 christos independently. Consider, for example, that the rule for the return to 287 1.1 christos standard time stayed the same from 1967 to 2006; but the rule for the 288 1.1 christos transition to daylight saving time changed several times in the same 289 1.1 christos period. There can also be periods, 1946 to 1966 for example, when no 290 1.1 christos rule from this group is in effect, and so either no transition 291 1.1 christos happened in those years, or some other rule is in effect (perhaps a 292 1.1 christos state or other more local rule).</li> 293 1.1 christos 294 1.1 christos <li>The <code>SAVE</code> and <code>LETTER</code> columns 295 1.1 christos contain <i>steady state</i>, not transitions. Consider, for example, 296 1.1 christos the transition from “war time” to “peace time” 297 1.1 christos that happened on August 14, 1945. The “1:00” in 298 1.1 christos the <code>SAVE</code> column is <i>not</i> an instruction to advance 299 1.1 christos the clock an hour. It means that clocks should <i>be</i> one hour 300 1.1 christos ahead of standard time, which they already are because of the previous 301 1.1 christos rule, so there should be no change.</li> 302 1.1 christos 303 1.1 christos </ul> 304 1.1 christos 305 1.1 christos <p>OK, now let’s look at a Zone record:</p> 306 1.1 christos 307 1.1 christos <table border="1"> 308 1.1 christos <tr> 309 1.1 christos <th colspan="5">From the Source File</th> 310 1.1 christos </tr> 311 1.1 christos <tr> 312 1.1 christos <td colspan="6" align="center"><table><tr><td> 313 1.1 christos <pre> 314 1.1 christos #Zone NAME GMTOFF RULES FORMAT [UNTIL] 315 1.1 christos Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 316 1.1 christos -6:00 US C%sT 1920 317 1.1 christos -6:00 Chicago C%sT 1936 Mar 1 2:00 318 1.1 christos -5:00 - EST 1936 Nov 15 2:00 319 1.1 christos -6:00 Chicago C%sT 1942 320 1.1 christos -6:00 US C%sT 1946 321 1.1 christos -6:00 Chicago C%sT 1967 322 1.1 christos -6:00 US C%sT 323 1.1 christos </pre> 324 1.1 christos </td></tr></table></td> 325 1.1 christos </tr> 326 1.1 christos <tr> 327 1.1 christos <th colspan="5">Columns Renamed</th> 328 1.1 christos </tr> 329 1.1 christos <tr> 330 1.1 christos <th rowspan="2">Standard Offset<br> 331 1.1 christos from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 332 1.1 christos Meridian</a></th> 333 1.1 christos <th rowspan="2">Daylight<br>Saving Time</th> 334 1.1 christos <th rowspan="2">Abbreviation(s)</th> 335 1.1 christos <th colspan="2">Ending at Local Time</th> 336 1.1 christos </tr> 337 1.1 christos <tr> 338 1.1 christos <th>Date</th> 339 1.1 christos <th>Time</th> 340 1.1 christos </tr> 341 1.1 christos <tr align="center"> 342 1.1 christos <td>−5:50:36</td> 343 1.1 christos <td>not observed</td> 344 1.1 christos <td>LMT</td> 345 1.1 christos <td>1883-11-18</td> 346 1.1 christos <td>12:09:24</td> 347 1.1 christos </tr> 348 1.1 christos <tr align="center"> 349 1.1 christos <td rowspan="2">−6:00:00</td> 350 1.1 christos <td>US rules</td> 351 1.1 christos <td rowspan="2">CST or CDT</td> 352 1.1 christos <td>1920-01-01</td> 353 1.1 christos <td>00:00:00</td> 354 1.1 christos </tr> 355 1.1 christos <tr align="center"> 356 1.1 christos <td>Chicago rules</td> 357 1.1 christos <td>1936-03-01</td> 358 1.1 christos <td rowspan="2">02:00:00</td> 359 1.1 christos </tr> 360 1.1 christos <tr align="center"> 361 1.1 christos <td>−5:00:00</td> 362 1.1 christos <td>not observed</td> 363 1.1 christos <td>EST</td> 364 1.1 christos <td>1936-11-15</td> 365 1.1 christos </tr> 366 1.1 christos <tr align="center"> 367 1.1 christos <td rowspan="4">−6:00:00</td> 368 1.1 christos <td>Chicago rules</td> 369 1.1 christos <td>CST or CDT</td> 370 1.1 christos <td>1942-01-01</td> 371 1.1 christos <td rowspan="3">00:00:00</td> 372 1.1 christos </tr> 373 1.1 christos <tr align="center"> 374 1.1 christos <td>US rules</td> 375 1.1 christos <td>CST, CWT or CPT</td> 376 1.1 christos <td>1946-01-01</td> 377 1.1 christos </tr> 378 1.1 christos <tr align="center"> 379 1.1 christos <td>Chicago rules</td> 380 1.1 christos <td rowspan="2">CST or CDT</td> 381 1.1 christos <td>1967-01-01</td> 382 1.1 christos </tr> 383 1.1 christos <tr align="center"> 384 1.1 christos <td>US rules</td> 385 1.1 christos <td colspan="2">—</td> 386 1.1 christos </tr> 387 1.1 christos </table> 388 1.1 christos 389 1.1 christos <p>There are a couple of interesting differences between Zones and Rules.</p> 390 1.1 christos 391 1.1 christos <p>First, and somewhat trivially, whereas Rules are considered to 392 1.1 christos contain one or more records, a Zone is considered to be a single 393 1.1 christos record with zero or more <i>continuation lines</i>. Thus, the keyword, 394 1.1 christos “<code>Zone</code>,” and the zone name are not 395 1.1 christos repeated. The last line is the one without anything in 396 1.1 christos the <code>[UNTIL]</code> column.</p> 397 1.1 christos 398 1.1 christos <p>Second, and more fundamentally, each line of a Zone represents a 399 1.1 christos steady state, not a transition between states. The state exists from 400 1.1 christos the date and time in the previous line’s <code>[UNTIL]</code> 401 1.1 christos column up to the date and time in the current 402 1.1 christos line’s <code>[UNTIL]</code> column. In other words, the date and 403 1.1 christos time in the <code>[UNTIL]</code> column is the instant that separates 404 1.1 christos this state from the next. Where that would be ambiguous because 405 1.1 christos we’re setting our clocks back, the <code>[UNTIL]</code> column 406 1.1 christos specifies the first occurrence of the instant. The state specified by 407 1.1 christos the last line, the one without anything in the <code>[UNTIL]</code> 408 1.1 christos column, continues to the present.</p> 409 1.1 christos 410 1.1 christos <p>The first line typically specifies the mean solar time observed 411 1.1 christos before the introduction of standard time. Since there’s no line before 412 1.1 christos that, it has no beginning. <code>8-) </code> For some places near the <a 413 1.1 christos href="https://en.wikipedia.org/wiki/International_Date_Line">International 414 1.1 christos Date Line</a>, the first <i>two</i> lines will show solar times 415 1.1 christos differing by 24 hours; this corresponds to a movement of the Date 416 1.1 christos Line. For example:</p> 417 1.1 christos 418 1.1 christos <pre> 419 1.1 christos #Zone NAME GMTOFF RULES FORMAT [UNTIL] 420 1.1 christos Zone America/Juneau 15:02:19 - LMT 1867 Oct 18 421 1.1 christos -8:57:41 - LMT ... 422 1.1 christos </pre> 423 1.1 christos 424 1.1 christos <p>When Alaska was purchased from Russia in 1867, the Date Line moved 425 1.1 christos from the Alaska/Canada border to the Bering Strait; and the time in 426 1.1 christos Alaska was then 24 hours earlier than it had 427 1.1 christos been. <code><aside></code>(6 October in the Julian calendar, 428 1.1 christos which Russia was still using then for religious reasons, was followed 429 1.1 christos by <i>a second instance of the same day with a different name</i>, 18 430 1.1 christos October in the Gregorian calendar. Isn’t civil time 431 1.1 christos wonderful? <code>8-)</code>)<code></aside></code></p> 432 1.1 christos 433 1.1 christos <p>The abbreviation, “LMT” stands for “local mean 434 1.1 christos time”, which is an invention of 435 1.1 christos the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 436 1.1 christos database</a> and was probably never actually used during the 437 1.1 christos period. Furthermore, the value is almost certainly wrong except in the 438 1.1 christos archetypal place after which the zone is named. (The tz database 439 1.1 christos usually doesn’t provide a separate Zone record for places where 440 1.1 christos nothing significant happened after 1970.)</p> 441 1.1 christos 442 1.1 christos <p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 443 1.1 christos <ul> 444 1.1 christos <li>A hyphen, a kind of null value, means that we have not set our 445 1.1 christos clocks ahead of standard time.</li> 446 1.1 christos 447 1.1 christos <li>An amount of time (usually but not necessarily “1:00” 448 1.1 christos meaning one hour) means that we have set our clocks ahead by that 449 1.1 christos amount.</li> 450 1.1 christos 451 1.1 christos <li>Some alphabetic string means that we <i>might have</i> set our 452 1.1 christos clocks ahead; and we need to check the rule the name of which is the 453 1.1 christos given alphabetic string.</li> 454 1.1 christos </ul> 455 1.1 christos 456 1.1 christos <p>An example of a specific amount of time is:</p> 457 1.1 christos <pre> 458 1.1 christos #Zone NAME GMTOFF RULES FORMAT [UNTIL] 459 1.1 christos Zone Pacific/Honolulu ... 1933 Apr 30 2:00 460 1.1 christos -10:30 1:00 HDT 1933 May 21 2:00 461 1.1 christos ... 462 1.1 christos </pre> 463 1.1 christos 464 1.1 christos <p>Hawaii tried daylight saving time for three weeks in 1933 and 465 1.1 christos decided they didn’t like it. <code>8-) </code>Note that 466 1.1 christos the <code>GMTOFF</code> column always contains the standard time 467 1.1 christos offset, so the wall clock time during this period was GMT − 468 1.1 christos 10:30 + 1:00 = GMT − 9:30.</p> 469 1.1 christos 470 1.1 christos <p>The <code>FORMAT</code> column specifies the usual abbreviation of 471 1.1 christos the time zone name. It can have one of three forms:</p> 472 1.1 christos <ul> 473 1.1 christos 474 1.1 christos <li>a string of three or more characters that are either ASCII alphanumerics, 475 1.1 christos “<code>+</code>”, or “<code>-</code>”, 476 1.1 christos in which case that’s the abbreviation</li> 477 1.1 christos 478 1.1 christos <li>a pair of strings separated by a slash 479 1.1 christos (‘<code>/</code>’), in which case the first string is the 480 1.1 christos abbreviation for the standard time name and the second string is the 481 1.1 christos abbreviation for the daylight saving time name</li> 482 1.1 christos 483 1.1 christos <li>a string containing “<code>%s</code>,” in which case 484 1.1 christos the “<code>%s</code>” will be replaced by the text in the 485 1.1 christos appropriate Rule’s <code>LETTER</code> column</li> 486 1.1 christos </ul> 487 1.1 christos 488 1.1 christos <p>The last two make sense only if there’s a named rule in effect.</p> 489 1.1 christos 490 1.1 christos <p>An example of a slash is:</p> 491 1.1 christos <pre> 492 1.1 christos #Zone NAME GMTOFF RULES FORMAT [UNTIL] 493 1.1 christos Zone Europe/London ... 1996 494 1.1 christos 0:00 EU GMT/BST 495 1.1 christos </pre> 496 1.1 christos 497 1.1 christos <p>The current time in the UK is called either Greenwich mean time or 498 1.1 christos British summer time.</p> 499 1.1 christos 500 1.1 christos <p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 501 1.1 christos happens when switching to a named rule. To what values should 502 1.1 christos the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 503 1.1 christos 504 1.1 christos <ul> 505 1.1 christos <li>If at least one transition has happened, use 506 1.1 christos the <code>SAVE</code> and <code>LETTER</code> data from the most 507 1.1 christos recent.</li> 508 1.1 christos 509 1.1 christos <li>If switching to a named rule before any transition has happened, 510 1.1 christos assume standard time (<code>SAVE</code> zero), and use 511 1.1 christos the <code>LETTER</code> data from the earliest transition with 512 1.1 christos a <code>SAVE</code> of zero. 513 1.1 christos 514 1.1 christos </ul> 515 1.1 christos 516 1.1 christos <p>And three last things about the <code>FORMAT</code> column:</p> 517 1.1 christos <ul> 518 1.1 christos 519 1.1 christos <li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 520 1.1 christos database</a> gives abbreviations for time zone names in <i>popular 521 1.1 christos usage</i>, which is not necessarily “correct” by law. For 522 1.1 christos example, the last line in 523 1.1 christos <code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 524 1.1 christos “HST” for “Hawaii standard time” even though the 525 1.3 christos <a href="https://www.law.cornell.edu/uscode/text/15/263">legal</a> 526 1.1 christos name for that time zone is “Hawaii-Aleutian standard time.” 527 1.1 christos This author has read that there are also some places in Australia where 528 1.1 christos popular time zone names differ from the legal ones. 529 1.1 christos 530 1.1 christos <li>No attempt is made to <a 531 1.1 christos href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 532 1.1 christos the abbreviations. They are intended to be the values returned through the 533 1.1 christos <code>"%Z"</code> format specifier to 534 1.1 christos <a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 535 1.1 christos <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 536 1.1 christos function in the 537 1.3 christos <a href="http://kirste.userpage.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 538 1.1 christos 539 1.1 christos <li>If there is no generally-accepted abbreviation for a time zone, 540 1.1 christos a numeric offset is used instead, e.g., <code>+07</code> for 7 hours 541 1.1 christos ahead of Greenwich. By convention, <code>-00</code> is used in a 542 1.1 christos zone while uninhabited, where the offset is zero but in some sense 543 1.1 christos the true offset is undefined. 544 1.1 christos </ul> 545 1.1 christos 546 1.1 christos <p>As a final example, here’s the complete history for Hawaii:</p> 547 1.1 christos 548 1.1 christos <table border="1"> 549 1.1 christos <tr> 550 1.1 christos <th colspan="6">Relevant Excerpts from the US Rules</th> 551 1.1 christos </tr> 552 1.1 christos <tr> 553 1.1 christos <td colspan="6" align="center"><table><tr><td> 554 1.1 christos <pre> 555 1.1 christos #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 556 1.1 christos Rule US 1918 1919 - Oct lastSun 2:00 0 S 557 1.1 christos Rule US 1942 only - Feb 9 2:00 1:00 W # War 558 1.1 christos Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 559 1.1 christos Rule US 1945 only - Sep 30 2:00 0 S 560 1.1 christos </pre> 561 1.1 christos </td></tr></table></td> 562 1.1 christos </tr> 563 1.1 christos <tr> 564 1.1 christos <th colspan="6">The Zone Record</th> 565 1.1 christos </tr> 566 1.1 christos <tr> 567 1.1 christos <td colspan="6" align="center"><table><tr><td> 568 1.1 christos <pre> 569 1.1 christos #Zone NAME GMTOFF RULES FORMAT [UNTIL] 570 1.1 christos Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 571 1.1 christos -10:30 - HST 1933 Apr 30 2:00 572 1.1 christos -10:30 1:00 HDT 1933 May 21 2:00 573 1.1 christos -10:30 US H%sT 1947 Jun 8 2:00 574 1.1 christos -10:00 - HST 575 1.1 christos </pre> 576 1.1 christos </td></tr></table></td> 577 1.1 christos </tr> 578 1.1 christos <tr> 579 1.1 christos <th colspan="6">What We Infer</th> 580 1.1 christos </tr> 581 1.1 christos <tr> 582 1.1 christos <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 583 1.1 christos <th rowspan="2">Adjust<br>Clocks</th> 584 1.1 christos <th colspan="2">Time Zone</th> 585 1.1 christos <th colspan="2">Ending at Local Time</th> 586 1.1 christos </tr> 587 1.1 christos <tr> 588 1.1 christos <th>Abbrv.</th> 589 1.1 christos <th>Name</th> 590 1.1 christos <th>Date</th> 591 1.1 christos <th>Time</th> 592 1.1 christos </tr> 593 1.1 christos <tr align="center"> 594 1.1 christos <td>−10:31:26</td> 595 1.1 christos <td>—</td> 596 1.1 christos <td>LMT</td> 597 1.1 christos <td>local mean time</td> 598 1.1 christos <td>1900-01-01</td> 599 1.1 christos <td>12:00</td> 600 1.1 christos </tr> 601 1.1 christos <tr align="center"> 602 1.1 christos <td>−10:30</td> 603 1.1 christos <td>+0:01:26</td> 604 1.1 christos <td>HST</td> 605 1.1 christos <td>Hawaii standard time</td> 606 1.1 christos <td>1933-04-30</td> 607 1.1 christos <td rowspan="3">02:00</td> 608 1.1 christos </tr> 609 1.1 christos <tr align="center"> 610 1.1 christos <td>−9:30</td> 611 1.1 christos <td>+1:00</td> 612 1.1 christos <td>HDT</td> 613 1.1 christos <td>Hawaii daylight time</td> 614 1.1 christos <td>1933-05-21</td> 615 1.1 christos </tr> 616 1.1 christos <tr align="center"> 617 1.1 christos <td>−10:30¹</td> 618 1.1 christos <td>−1:00¹</td> 619 1.1 christos <td>HST¹</td> 620 1.1 christos <td>Hawaii standard time</td> 621 1.1 christos <td>1942-02-09</td> 622 1.1 christos </tr> 623 1.1 christos <tr align="center"> 624 1.1 christos <td rowspan="2">−9:30</td> 625 1.1 christos <td>+1:00</td> 626 1.1 christos <td>HWT</td> 627 1.1 christos <td>Hawaii war time</td> 628 1.1 christos <td>1945-08-14</td> 629 1.1 christos <td>13:30²</td> 630 1.1 christos </tr> 631 1.1 christos <tr align="center"> 632 1.1 christos <td>0</td> 633 1.1 christos <td>HPT</td> 634 1.1 christos <td>Hawaii peace time</td> 635 1.1 christos <td>1945-09-30</td> 636 1.1 christos <td rowspan="2">02:00</td> 637 1.1 christos </tr> 638 1.1 christos <tr align="center"> 639 1.1 christos <td>−10:30</td> 640 1.1 christos <td>−1:00</td> 641 1.1 christos <td rowspan="2">HST</td> 642 1.1 christos <td rowspan="2">Hawaii standard time</td> 643 1.1 christos <td>1947-06-08</td> 644 1.1 christos </tr> 645 1.1 christos <tr align="center"> 646 1.1 christos <td>−10:00³</td> 647 1.1 christos <td>+0:30³</td> 648 1.1 christos <td colspan="2">—</td> 649 1.1 christos </tr> 650 1.1 christos <tr> 651 1.1 christos <td colspan="6"> 652 1.1 christos ¹Switching to US rules…most recent transition (in 1919) was to standard time 653 1.1 christos </td> 654 1.1 christos </tr> 655 1.1 christos <tr> 656 1.1 christos <td colspan="6"> 657 1.1 christos ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 658 1.1 christos + (−9:30) = 13:30 local 659 1.1 christos </td> 660 1.1 christos </tr> 661 1.1 christos <tr> 662 1.1 christos <td colspan="6"> 663 1.1 christos ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 664 1.1 christos the civil time in Hawaii has been 665 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> 666 1.1 christos − 10:00 year-round. 667 1.1 christos </td> 668 1.1 christos </tr> 669 1.1 christos </table> 670 1.1 christos 671 1.1 christos <p>There will be a short quiz later. <code>8-)</code></p> 672 1.1 christos 673 1.1 christos <hr> 674 1.1 christos <address> 675 1.1 christos This web page is in the public domain, so clarified as of 676 1.1 christos 2015-10-20 by Bill Seymour. 677 1.1 christos <br> 678 1.1 christos All suggestions and corrections will be welcome; all flames will be amusing. 679 1.1 christos Mail to was at pobox dot com. 680 1.1 christos </address> 681 1.1 christos </body> 682 1.1 christos </html> 683
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