tz-how-to.html revision 1.2
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 1.1 christos <head><title>How to Read the tz Database</title></head> 6 1.1 christos <body> 7 1.1 christos <h2>How to Read the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 8 1.1 christos Database</a> Source Files</h2> 9 1.1 christos <h3>by Bill Seymour</h3> 10 1.1 christos <p>This page uses the <code>America/Chicago</code> and 11 1.1 christos <code>Pacific/Honolulu</code> zones as examples of how to infer 12 1.1 christos times of day from the <a href="tz-link.htm">tz database</a> 13 1.1 christos source files. It might be helpful, but not absolutely necessary, 14 1.1 christos for the reader to have already downloaded the 15 1.2 christos latest release of the database and become familiar with the basic layout 16 1.1 christos of the data files. The format is explained in the “man 17 1.1 christos page” for the zic compiler, <code>zic.8.txt</code>, in 18 1.1 christos the <code>code</code> subdirectory.</p> 19 1.1 christos 20 1.1 christos <p>We’ll begin by talking about the rules for changing between standard 21 1.1 christos and daylight saving time since we’ll need that information when we talk 22 1.1 christos about the zones.</p> 23 1.1 christos 24 1.1 christos <p>First, let’s consider the special daylight saving time rules 25 1.1 christos for Chicago (from the <code>northamerica</code> file in 26 1.1 christos the <code>data</code> subdirectory):</p> 27 1.1 christos 28 1.1 christos <table border="1"> 29 1.1 christos <tr> 30 1.1 christos <th colspan="6">From the Source File</th> 31 1.1 christos </tr> 32 1.1 christos <tr> 33 1.1 christos <td colspan="6" align="center"><table><tr><td> 34 1.1 christos <pre> 35 1.1 christos #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER 36 1.1 christos Rule Chicago 1920 only - Jun 13 2:00 1:00 D 37 1.1 christos Rule Chicago 1920 1921 - Oct lastSun 2:00 0 S 38 1.1 christos Rule Chicago 1921 only - Mar lastSun 2:00 1:00 D 39 1.1 christos Rule Chicago 1922 1966 - Apr lastSun 2:00 1:00 D 40 1.1 christos Rule Chicago 1922 1954 - Sep lastSun 2:00 0 S 41 1.1 christos Rule Chicago 1955 1966 - Oct lastSun 2:00 0 S 42 1.1 christos </pre> 43 1.1 christos </td></tr></table></td> 44 1.1 christos </tr> 45 1.1 christos <tr> 46 1.1 christos <th colspan="6">Reformatted a Bit</th> 47 1.1 christos </tr> 48 1.1 christos <tr> 49 1.1 christos <th>From</th> 50 1.1 christos <th>To</th> 51 1.1 christos <th colspan="2">On</th> 52 1.1 christos <th>At</th> 53 1.1 christos <th>Action</th> 54 1.1 christos </tr> 55 1.1 christos <tr align="center"> 56 1.1 christos <td colspan="2">1920 only</td> 57 1.1 christos <td colspan="2">June 13<small><sup>th</sup></small></td> 58 1.1 christos <td rowspan="6">02:00 local</td> 59 1.1 christos <td>go to daylight saving time</td> 60 1.1 christos </tr> 61 1.1 christos <tr align="center"> 62 1.1 christos <td>1920</td> 63 1.1 christos <td>1921</td> 64 1.1 christos <td rowspan="5">last Sunday</td> 65 1.1 christos <td>in October</td> 66 1.1 christos <td>return to standard time</td> 67 1.1 christos </tr> 68 1.1 christos <tr align="center"> 69 1.1 christos <td colspan="2">1921 only</td> 70 1.1 christos <td>in March</td> 71 1.1 christos <td rowspan="2">go to daylight saving time</td> 72 1.1 christos </tr> 73 1.1 christos <tr align="center"> 74 1.1 christos <td rowspan="2">1922</td> 75 1.1 christos <td>1966</td> 76 1.1 christos <td>in April</td> 77 1.1 christos </tr> 78 1.1 christos <tr align="center"> 79 1.1 christos <td>1954</td> 80 1.1 christos <td>in September</td> 81 1.1 christos <td rowspan="2">return to standard time</td> 82 1.1 christos </tr> 83 1.1 christos <tr align="center"> 84 1.1 christos <td>1955</td> 85 1.1 christos <td>1966</td> 86 1.1 christos <td>in October</td> 87 1.1 christos </tr> 88 1.1 christos </table> 89 1.1 christos 90 1.1 christos <p>We’ll basically just ignore the <code>TYPE</code> column. 91 1.1 christos In the 2007j release, the most recent as of this writing, the 92 1.1 christos <code>TYPE</code> column never contains anything but a hyphen, 93 1.1 christos a kind of null value. (From the description in <code>zic.8.txt</code>, 94 1.1 christos this appears to be a mechanism for removing years from a set 95 1.1 christos in some localizable way. It’s used in the file, <code>pacificnew</code>, 96 1.1 christos to determine whether a given year will have a US presidential election; 97 1.1 christos but everything related to that use is commented out.) 98 1.1 christos 99 1.1 christos <p>The <code>SAVE</code> column contains the wall clock offset from 100 1.1 christos local standard time. 101 1.1 christos This is usually either zero for standard time or one hour for daylight 102 1.1 christos saving time; but there’s no reason, in principle, why it can’t 103 1.1 christos take on other values. 104 1.1 christos 105 1.1 christos <p>The <code>LETTER</code> (sometimes called <code>LETTER/S</code>) 106 1.1 christos column can contain a variable 107 1.1 christos part of the usual abbreviation of the time zone’s name, or it can just 108 1.1 christos be a hyphen if there’s no variable part. For example, the abbreviation 109 1.1 christos used in the central time zone will be either “CST” or 110 1.1 christos “CDT”. The variable part is ‘S’ or ‘D’; 111 1.1 christos and, sure enough, that’s just what we find in 112 1.1 christos the <code>LETTER</code> column 113 1.1 christos in the <code>Chicago</code> rules. More about this when we talk about 114 1.1 christos “Zone” lines. 115 1.1 christos 116 1.1 christos <p>One important thing to notice is that “Rule” lines 117 1.1 christos want at once to be both <i>transitions</i> and <i>steady states</i>: 118 1.1 christos <ul> 119 1.1 christos <li>On the one hand, they represent transitions between standard and 120 1.1 christos daylight saving time; and any number of Rule lines can be in effect 121 1.1 christos during a given period (which will always be a non-empty set of 122 1.1 christos contiguous calendar years).</li> 123 1.1 christos <li>On the other hand, the <code>SAVE</code> and <code>LETTER</code> 124 1.1 christos columns contain state that exists between transitions. More about this 125 1.1 christos when we talk about the US rules.</li> 126 1.1 christos </ul> 127 1.1 christos 128 1.1 christos <p>In the example above, the transition to daylight saving time 129 1.1 christos happened on the 13<small><sup>th</sup></small> of June in 1920, and on 130 1.1 christos the last Sunday in March in 1921; but the return to standard time 131 1.1 christos happened on the last Sunday in October in both of those 132 1.1 christos years. Similarly, the rule for changing to daylight saving time was 133 1.1 christos the same from 1922 to 1966; but the rule for returning to standard 134 1.1 christos time changed in 1955. Got it?</p> 135 1.1 christos 136 1.1 christos <p>OK, now for the somewhat more interesting “US” rules:</p> 137 1.1 christos 138 1.1 christos <table border="1"> 139 1.1 christos <tr> 140 1.1 christos <th colspan="6">From the Source File</th> 141 1.1 christos </tr> 142 1.1 christos <tr> 143 1.1 christos <td colspan="6" align="center"><table><tr><td> 144 1.1 christos <pre> 145 1.1 christos #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 146 1.1 christos Rule US 1918 1919 - Mar lastSun 2:00 1:00 D 147 1.1 christos Rule US 1918 1919 - Oct lastSun 2:00 0 S 148 1.1 christos Rule US 1942 only - Feb 9 2:00 1:00 W # War 149 1.1 christos Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 150 1.1 christos Rule US 1945 only - Sep 30 2:00 0 S 151 1.1 christos Rule US 1967 2006 - Oct lastSun 2:00 0 S 152 1.1 christos Rule US 1967 1973 - Apr lastSun 2:00 1:00 D 153 1.1 christos Rule US 1974 only - Jan 6 2:00 1:00 D 154 1.1 christos Rule US 1975 only - Feb 23 2:00 1:00 D 155 1.1 christos Rule US 1976 1986 - Apr lastSun 2:00 1:00 D 156 1.1 christos Rule US 1987 2006 - Apr Sun>=1 2:00 1:00 D 157 1.1 christos Rule US 2007 max - Mar Sun>=8 2:00 1:00 D 158 1.1 christos Rule US 2007 max - Nov Sun>=1 2:00 0 S 159 1.1 christos </pre> 160 1.1 christos </td></tr></table></td> 161 1.1 christos </tr> 162 1.1 christos <tr> 163 1.1 christos <th colspan="6">Reformatted a Bit</th> 164 1.1 christos </tr> 165 1.1 christos <tr> 166 1.1 christos <th>From</th> 167 1.1 christos <th>To</th> 168 1.1 christos <th colspan="2">On</th> 169 1.1 christos <th>At</th> 170 1.1 christos <th>Action</th> 171 1.1 christos </tr> 172 1.1 christos <tr align="center"> 173 1.1 christos <td rowspan="2">1918</td> 174 1.1 christos <td rowspan="2">1919</td> 175 1.1 christos <td rowspan="2">last Sunday</td> 176 1.1 christos <td>in March</td> 177 1.1 christos <td rowspan="3">02:00 local</td> 178 1.1 christos <td>go to daylight saving time</td> 179 1.1 christos </tr> 180 1.1 christos <tr align="center"> 181 1.1 christos <td>in October</td> 182 1.1 christos <td>return to standard time</td> 183 1.1 christos </tr> 184 1.1 christos <tr align="center"> 185 1.1 christos <td colspan="2">1942 only</td> 186 1.1 christos <td colspan="2">February 9<small><sup>th</sup></small></td> 187 1.1 christos <td>go to “war time”</td> 188 1.1 christos </tr> 189 1.1 christos <tr align="center"> 190 1.1 christos <td colspan="2" rowspan="2">1945 only</td> 191 1.1 christos <td colspan="2">August 14<small><sup>th</sup></small></td> 192 1.1 christos <td>23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a></td> 193 1.1 christos <td> 194 1.1 christos rename “war time” to “peace<br>time;” 195 1.1 christos clocks don’t change 196 1.1 christos </td> 197 1.1 christos </tr> 198 1.1 christos <tr align="center"> 199 1.1 christos <td colspan="2">September 30<small><sup>th</sup></small></td> 200 1.1 christos <td rowspan="9">02:00 local</td> 201 1.1 christos <td rowspan="2">return to standard time</td> 202 1.1 christos </tr> 203 1.1 christos <tr align="center"> 204 1.1 christos <td rowspan="2">1967</td> 205 1.1 christos <td>2006</td> 206 1.1 christos <td rowspan="2">last Sunday</td> 207 1.1 christos <td>in October</td> 208 1.1 christos </tr> 209 1.1 christos <tr align="center"> 210 1.1 christos <td>1973</td> 211 1.1 christos <td>in April</td> 212 1.1 christos <td rowspan="6">go to daylight saving time</td> 213 1.1 christos </tr> 214 1.1 christos <tr align="center"> 215 1.1 christos <td colspan="2">1974 only</td> 216 1.1 christos <td colspan="2">January 6<small><sup>th</sup></small></td> 217 1.1 christos </tr> 218 1.1 christos <tr align="center"> 219 1.1 christos <td colspan="2">1975 only</td> 220 1.1 christos <td colspan="2">February 23<small><sup>rd</sup></small></td> 221 1.1 christos </tr> 222 1.1 christos <tr align="center"> 223 1.1 christos <td>1976</td> 224 1.1 christos <td>1986</td> 225 1.1 christos <td>last Sunday</td> 226 1.1 christos <td rowspan="2">in April</td> 227 1.1 christos </tr> 228 1.1 christos <tr align="center"> 229 1.1 christos <td>1987</td> 230 1.1 christos <td>2006</td> 231 1.1 christos <td>first Sunday</td> 232 1.1 christos </tr> 233 1.1 christos <tr align="center"> 234 1.1 christos <td rowspan="2">2007</td> 235 1.1 christos <td rowspan="2">present</td> 236 1.1 christos <td colspan="2">second Sunday in March</td> 237 1.1 christos </tr> 238 1.1 christos <tr align="center"> 239 1.1 christos <td colspan="2">first Sunday in November</td> 240 1.1 christos <td>return to standard time</td> 241 1.1 christos </tr> 242 1.1 christos </table> 243 1.1 christos 244 1.1 christos <p>There are two interesting things to note here.</p> 245 1.1 christos 246 1.1 christos <p>First, the time that something happens (in the <code>AT</code> 247 1.1 christos column) is not necessarily the local wall clock time. The time can be 248 1.1 christos suffixed with ‘s’ (for “standard”) to mean 249 1.1 christos local standard time (different from wall clock time when observing 250 1.1 christos daylight saving time); or it can be suffixed with ‘g’, 251 1.1 christos ‘u’, or ‘z’, all three of which mean the 252 1.1 christos standard time at the 253 1.2 christos <a href="https://en.wikipedia.org/wiki/Prime_Meridian">prime meridian</a>. 254 1.1 christos ‘g’ stands for “<a 255 1.1 christos href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">GMT</a>”; 256 1.1 christos ‘u’ stands for “<a 257 1.1 christos href="https://en.wikipedia.org/wiki/Universal_Time">UT</a>” or “<a 258 1.1 christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">UTC</a>” 259 1.1 christos (whichever was official at the time); ‘z’ stands for the 260 1.1 christos <a href="https://en.wikipedia.org/wiki/Nautical_time">nautical time zone</a> 261 1.1 christos Z (a.k.a. “Zulu” which, in turn, stands for ‘Z’). 262 1.1 christos The time can also be suffixed with ‘w’ meaning “wall 263 1.1 christos clock time;” but it usually isn’t because that’s the 264 1.1 christos default.</p> 265 1.1 christos 266 1.1 christos <p>Second, the day in the <code>ON</code> column, in addition to 267 1.1 christos “<code>lastSun</code>” or a particular day of the month, 268 1.1 christos can have the form, “<code>Sun>=</code><i>x</i>” or 269 1.1 christos “<code>Sun<=</code><i>x</i>,” where <i>x</i> is a day 270 1.1 christos of the month. For example, “<code>Sun>=8</code>” means 271 1.1 christos “the first Sunday on or after the eighth of the month,” in 272 1.1 christos other words, the second Sunday of the month. Furthermore, although 273 1.1 christos there are no examples above, the weekday needn’t be 274 1.1 christos “<code>Sun</code>” in either form, but can be the usual 275 1.1 christos three-character English abbreviation for any day of the week.</p> 276 1.1 christos 277 1.1 christos <p>And the US rules give us more examples of a couple of things 278 1.1 christos already mentioned:</p> 279 1.1 christos 280 1.1 christos <ul> 281 1.1 christos <li>The rules for changing to and from daylight saving time are 282 1.1 christos actually <i>different sets</i> of rules; and the two sets can change 283 1.1 christos independently. Consider, for example, that the rule for the return to 284 1.1 christos standard time stayed the same from 1967 to 2006; but the rule for the 285 1.1 christos transition to daylight saving time changed several times in the same 286 1.1 christos period. There can also be periods, 1946 to 1966 for example, when no 287 1.1 christos rule from this group is in effect, and so either no transition 288 1.1 christos happened in those years, or some other rule is in effect (perhaps a 289 1.1 christos state or other more local rule).</li> 290 1.1 christos 291 1.1 christos <li>The <code>SAVE</code> and <code>LETTER</code> columns 292 1.1 christos contain <i>steady state</i>, not transitions. Consider, for example, 293 1.1 christos the transition from “war time” to “peace time” 294 1.1 christos that happened on August 14, 1945. The “1:00” in 295 1.1 christos the <code>SAVE</code> column is <i>not</i> an instruction to advance 296 1.1 christos the clock an hour. It means that clocks should <i>be</i> one hour 297 1.1 christos ahead of standard time, which they already are because of the previous 298 1.1 christos rule, so there should be no change.</li> 299 1.1 christos 300 1.1 christos </ul> 301 1.1 christos 302 1.1 christos <p>OK, now let’s look at a Zone record:</p> 303 1.1 christos 304 1.1 christos <table border="1"> 305 1.1 christos <tr> 306 1.1 christos <th colspan="5">From the Source File</th> 307 1.1 christos </tr> 308 1.1 christos <tr> 309 1.1 christos <td colspan="6" align="center"><table><tr><td> 310 1.1 christos <pre> 311 1.1 christos #Zone NAME GMTOFF RULES FORMAT [UNTIL] 312 1.1 christos Zone America/Chicago -5:50:36 - LMT 1883 Nov 18 12:09:24 313 1.1 christos -6:00 US C%sT 1920 314 1.1 christos -6:00 Chicago C%sT 1936 Mar 1 2:00 315 1.1 christos -5:00 - EST 1936 Nov 15 2:00 316 1.1 christos -6:00 Chicago C%sT 1942 317 1.1 christos -6:00 US C%sT 1946 318 1.1 christos -6:00 Chicago C%sT 1967 319 1.1 christos -6:00 US C%sT 320 1.1 christos </pre> 321 1.1 christos </td></tr></table></td> 322 1.1 christos </tr> 323 1.1 christos <tr> 324 1.1 christos <th colspan="5">Columns Renamed</th> 325 1.1 christos </tr> 326 1.1 christos <tr> 327 1.1 christos <th rowspan="2">Standard Offset<br> 328 1.1 christos from <a href="https://en.wikipedia.org/wiki/Prime_Meridian">Prime 329 1.1 christos Meridian</a></th> 330 1.1 christos <th rowspan="2">Daylight<br>Saving Time</th> 331 1.1 christos <th rowspan="2">Abbreviation(s)</th> 332 1.1 christos <th colspan="2">Ending at Local Time</th> 333 1.1 christos </tr> 334 1.1 christos <tr> 335 1.1 christos <th>Date</th> 336 1.1 christos <th>Time</th> 337 1.1 christos </tr> 338 1.1 christos <tr align="center"> 339 1.1 christos <td>−5:50:36</td> 340 1.1 christos <td>not observed</td> 341 1.1 christos <td>LMT</td> 342 1.1 christos <td>1883-11-18</td> 343 1.1 christos <td>12:09:24</td> 344 1.1 christos </tr> 345 1.1 christos <tr align="center"> 346 1.1 christos <td rowspan="2">−6:00:00</td> 347 1.1 christos <td>US rules</td> 348 1.1 christos <td rowspan="2">CST or CDT</td> 349 1.1 christos <td>1920-01-01</td> 350 1.1 christos <td>00:00:00</td> 351 1.1 christos </tr> 352 1.1 christos <tr align="center"> 353 1.1 christos <td>Chicago rules</td> 354 1.1 christos <td>1936-03-01</td> 355 1.1 christos <td rowspan="2">02:00:00</td> 356 1.1 christos </tr> 357 1.1 christos <tr align="center"> 358 1.1 christos <td>−5:00:00</td> 359 1.1 christos <td>not observed</td> 360 1.1 christos <td>EST</td> 361 1.1 christos <td>1936-11-15</td> 362 1.1 christos </tr> 363 1.1 christos <tr align="center"> 364 1.1 christos <td rowspan="4">−6:00:00</td> 365 1.1 christos <td>Chicago rules</td> 366 1.1 christos <td>CST or CDT</td> 367 1.1 christos <td>1942-01-01</td> 368 1.1 christos <td rowspan="3">00:00:00</td> 369 1.1 christos </tr> 370 1.1 christos <tr align="center"> 371 1.1 christos <td>US rules</td> 372 1.1 christos <td>CST, CWT or CPT</td> 373 1.1 christos <td>1946-01-01</td> 374 1.1 christos </tr> 375 1.1 christos <tr align="center"> 376 1.1 christos <td>Chicago rules</td> 377 1.1 christos <td rowspan="2">CST or CDT</td> 378 1.1 christos <td>1967-01-01</td> 379 1.1 christos </tr> 380 1.1 christos <tr align="center"> 381 1.1 christos <td>US rules</td> 382 1.1 christos <td colspan="2">—</td> 383 1.1 christos </tr> 384 1.1 christos </table> 385 1.1 christos 386 1.1 christos <p>There are a couple of interesting differences between Zones and Rules.</p> 387 1.1 christos 388 1.1 christos <p>First, and somewhat trivially, whereas Rules are considered to 389 1.1 christos contain one or more records, a Zone is considered to be a single 390 1.1 christos record with zero or more <i>continuation lines</i>. Thus, the keyword, 391 1.1 christos “<code>Zone</code>,” and the zone name are not 392 1.1 christos repeated. The last line is the one without anything in 393 1.1 christos the <code>[UNTIL]</code> column.</p> 394 1.1 christos 395 1.1 christos <p>Second, and more fundamentally, each line of a Zone represents a 396 1.1 christos steady state, not a transition between states. The state exists from 397 1.1 christos the date and time in the previous line’s <code>[UNTIL]</code> 398 1.1 christos column up to the date and time in the current 399 1.1 christos line’s <code>[UNTIL]</code> column. In other words, the date and 400 1.1 christos time in the <code>[UNTIL]</code> column is the instant that separates 401 1.1 christos this state from the next. Where that would be ambiguous because 402 1.1 christos we’re setting our clocks back, the <code>[UNTIL]</code> column 403 1.1 christos specifies the first occurrence of the instant. The state specified by 404 1.1 christos the last line, the one without anything in the <code>[UNTIL]</code> 405 1.1 christos column, continues to the present.</p> 406 1.1 christos 407 1.1 christos <p>The first line typically specifies the mean solar time observed 408 1.1 christos before the introduction of standard time. Since there’s no line before 409 1.1 christos that, it has no beginning. <code>8-) </code> For some places near the <a 410 1.1 christos href="https://en.wikipedia.org/wiki/International_Date_Line">International 411 1.1 christos Date Line</a>, the first <i>two</i> lines will show solar times 412 1.1 christos differing by 24 hours; this corresponds to a movement of the Date 413 1.1 christos Line. For example:</p> 414 1.1 christos 415 1.1 christos <pre> 416 1.1 christos #Zone NAME GMTOFF RULES FORMAT [UNTIL] 417 1.1 christos Zone America/Juneau 15:02:19 - LMT 1867 Oct 18 418 1.1 christos -8:57:41 - LMT ... 419 1.1 christos </pre> 420 1.1 christos 421 1.1 christos <p>When Alaska was purchased from Russia in 1867, the Date Line moved 422 1.1 christos from the Alaska/Canada border to the Bering Strait; and the time in 423 1.1 christos Alaska was then 24 hours earlier than it had 424 1.1 christos been. <code><aside></code>(6 October in the Julian calendar, 425 1.1 christos which Russia was still using then for religious reasons, was followed 426 1.1 christos by <i>a second instance of the same day with a different name</i>, 18 427 1.1 christos October in the Gregorian calendar. Isn’t civil time 428 1.1 christos wonderful? <code>8-)</code>)<code></aside></code></p> 429 1.1 christos 430 1.1 christos <p>The abbreviation, “LMT” stands for “local mean 431 1.1 christos time”, which is an invention of 432 1.1 christos the <a href="https://en.wikipedia.org/wiki/Tz_database">tz 433 1.1 christos database</a> and was probably never actually used during the 434 1.1 christos period. Furthermore, the value is almost certainly wrong except in the 435 1.1 christos archetypal place after which the zone is named. (The tz database 436 1.1 christos usually doesn’t provide a separate Zone record for places where 437 1.1 christos nothing significant happened after 1970.)</p> 438 1.1 christos 439 1.1 christos <p>The <code>RULES</code> column tells us whether daylight saving time is being observed: 440 1.1 christos <ul> 441 1.1 christos <li>A hyphen, a kind of null value, means that we have not set our 442 1.1 christos clocks ahead of standard time.</li> 443 1.1 christos 444 1.1 christos <li>An amount of time (usually but not necessarily “1:00” 445 1.1 christos meaning one hour) means that we have set our clocks ahead by that 446 1.1 christos amount.</li> 447 1.1 christos 448 1.1 christos <li>Some alphabetic string means that we <i>might have</i> set our 449 1.1 christos clocks ahead; and we need to check the rule the name of which is the 450 1.1 christos given alphabetic string.</li> 451 1.1 christos </ul> 452 1.1 christos 453 1.1 christos <p>An example of a specific amount of time is:</p> 454 1.1 christos <pre> 455 1.1 christos #Zone NAME GMTOFF RULES FORMAT [UNTIL] 456 1.1 christos Zone Pacific/Honolulu ... 1933 Apr 30 2:00 457 1.1 christos -10:30 1:00 HDT 1933 May 21 2:00 458 1.1 christos ... 459 1.1 christos </pre> 460 1.1 christos 461 1.1 christos <p>Hawaii tried daylight saving time for three weeks in 1933 and 462 1.1 christos decided they didn’t like it. <code>8-) </code>Note that 463 1.1 christos the <code>GMTOFF</code> column always contains the standard time 464 1.1 christos offset, so the wall clock time during this period was GMT − 465 1.1 christos 10:30 + 1:00 = GMT − 9:30.</p> 466 1.1 christos 467 1.1 christos <p>The <code>FORMAT</code> column specifies the usual abbreviation of 468 1.1 christos the time zone name. It can have one of three forms:</p> 469 1.1 christos <ul> 470 1.1 christos 471 1.1 christos <li>a string of three or more characters that are either ASCII alphanumerics, 472 1.1 christos “<code>+</code>”, or “<code>-</code>”, 473 1.1 christos in which case that’s the abbreviation</li> 474 1.1 christos 475 1.1 christos <li>a pair of strings separated by a slash 476 1.1 christos (‘<code>/</code>’), in which case the first string is the 477 1.1 christos abbreviation for the standard time name and the second string is the 478 1.1 christos abbreviation for the daylight saving time name</li> 479 1.1 christos 480 1.1 christos <li>a string containing “<code>%s</code>,” in which case 481 1.1 christos the “<code>%s</code>” will be replaced by the text in the 482 1.1 christos appropriate Rule’s <code>LETTER</code> column</li> 483 1.1 christos </ul> 484 1.1 christos 485 1.1 christos <p>The last two make sense only if there’s a named rule in effect.</p> 486 1.1 christos 487 1.1 christos <p>An example of a slash is:</p> 488 1.1 christos <pre> 489 1.1 christos #Zone NAME GMTOFF RULES FORMAT [UNTIL] 490 1.1 christos Zone Europe/London ... 1996 491 1.1 christos 0:00 EU GMT/BST 492 1.1 christos </pre> 493 1.1 christos 494 1.1 christos <p>The current time in the UK is called either Greenwich mean time or 495 1.1 christos British summer time.</p> 496 1.1 christos 497 1.1 christos <p>One wrinkle, not fully explained in <code>zic.8.txt</code>, is what 498 1.1 christos happens when switching to a named rule. To what values should 499 1.1 christos the <code>SAVE</code> and <code>LETTER</code> data be initialized?</p> 500 1.1 christos 501 1.1 christos <ul> 502 1.1 christos <li>If at least one transition has happened, use 503 1.1 christos the <code>SAVE</code> and <code>LETTER</code> data from the most 504 1.1 christos recent.</li> 505 1.1 christos 506 1.1 christos <li>If switching to a named rule before any transition has happened, 507 1.1 christos assume standard time (<code>SAVE</code> zero), and use 508 1.1 christos the <code>LETTER</code> data from the earliest transition with 509 1.1 christos a <code>SAVE</code> of zero. 510 1.1 christos 511 1.1 christos </ul> 512 1.1 christos 513 1.1 christos <p>And three last things about the <code>FORMAT</code> column:</p> 514 1.1 christos <ul> 515 1.1 christos 516 1.1 christos <li>The <a href="https://en.wikipedia.org/wiki/Tz_database">tz 517 1.1 christos database</a> gives abbreviations for time zone names in <i>popular 518 1.1 christos usage</i>, which is not necessarily “correct” by law. For 519 1.1 christos example, the last line in 520 1.1 christos <code>Zone</code> <code>Pacific/Honolulu</code> (shown below) gives 521 1.1 christos “HST” for “Hawaii standard time” even though the 522 1.1 christos <a href="http://www.law.cornell.edu/uscode/html/uscode15/usc_sec_15_00000263----000-.html">legal</a> 523 1.1 christos name for that time zone is “Hawaii-Aleutian standard time.” 524 1.1 christos This author has read that there are also some places in Australia where 525 1.1 christos popular time zone names differ from the legal ones. 526 1.1 christos 527 1.1 christos <li>No attempt is made to <a 528 1.1 christos href="https://en.wikipedia.org/wiki/Internationalization_and_localization">localize</a> 529 1.1 christos the abbreviations. They are intended to be the values returned through the 530 1.1 christos <code>"%Z"</code> format specifier to 531 1.1 christos <a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>’s 532 1.1 christos <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html"><code>strftime</code></a> 533 1.1 christos function in the 534 1.1 christos <a href="http://www.chemie.fu-berlin.de/chemnet/use/info/libc/libc_19.html#SEC324">“C” locale</a>. 535 1.1 christos 536 1.1 christos <li>If there is no generally-accepted abbreviation for a time zone, 537 1.1 christos a numeric offset is used instead, e.g., <code>+07</code> for 7 hours 538 1.1 christos ahead of Greenwich. By convention, <code>-00</code> is used in a 539 1.1 christos zone while uninhabited, where the offset is zero but in some sense 540 1.1 christos the true offset is undefined. 541 1.1 christos </ul> 542 1.1 christos 543 1.1 christos <p>As a final example, here’s the complete history for Hawaii:</p> 544 1.1 christos 545 1.1 christos <table border="1"> 546 1.1 christos <tr> 547 1.1 christos <th colspan="6">Relevant Excerpts from the US Rules</th> 548 1.1 christos </tr> 549 1.1 christos <tr> 550 1.1 christos <td colspan="6" align="center"><table><tr><td> 551 1.1 christos <pre> 552 1.1 christos #Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S 553 1.1 christos Rule US 1918 1919 - Oct lastSun 2:00 0 S 554 1.1 christos Rule US 1942 only - Feb 9 2:00 1:00 W # War 555 1.1 christos Rule US 1945 only - Aug 14 23:00u 1:00 P # Peace 556 1.1 christos Rule US 1945 only - Sep 30 2:00 0 S 557 1.1 christos </pre> 558 1.1 christos </td></tr></table></td> 559 1.1 christos </tr> 560 1.1 christos <tr> 561 1.1 christos <th colspan="6">The Zone Record</th> 562 1.1 christos </tr> 563 1.1 christos <tr> 564 1.1 christos <td colspan="6" align="center"><table><tr><td> 565 1.1 christos <pre> 566 1.1 christos #Zone NAME GMTOFF RULES FORMAT [UNTIL] 567 1.1 christos Zone Pacific/Honolulu -10:31:26 - LMT 1900 Jan 1 12:00 568 1.1 christos -10:30 - HST 1933 Apr 30 2:00 569 1.1 christos -10:30 1:00 HDT 1933 May 21 2:00 570 1.1 christos -10:30 US H%sT 1947 Jun 8 2:00 571 1.1 christos -10:00 - HST 572 1.1 christos </pre> 573 1.1 christos </td></tr></table></td> 574 1.1 christos </tr> 575 1.1 christos <tr> 576 1.1 christos <th colspan="6">What We Infer</th> 577 1.1 christos </tr> 578 1.1 christos <tr> 579 1.1 christos <th rowspan="2">Wall-Clock<br>Offset from<br>Prime Meridian</th> 580 1.1 christos <th rowspan="2">Adjust<br>Clocks</th> 581 1.1 christos <th colspan="2">Time Zone</th> 582 1.1 christos <th colspan="2">Ending at Local Time</th> 583 1.1 christos </tr> 584 1.1 christos <tr> 585 1.1 christos <th>Abbrv.</th> 586 1.1 christos <th>Name</th> 587 1.1 christos <th>Date</th> 588 1.1 christos <th>Time</th> 589 1.1 christos </tr> 590 1.1 christos <tr align="center"> 591 1.1 christos <td>−10:31:26</td> 592 1.1 christos <td>—</td> 593 1.1 christos <td>LMT</td> 594 1.1 christos <td>local mean time</td> 595 1.1 christos <td>1900-01-01</td> 596 1.1 christos <td>12:00</td> 597 1.1 christos </tr> 598 1.1 christos <tr align="center"> 599 1.1 christos <td>−10:30</td> 600 1.1 christos <td>+0:01:26</td> 601 1.1 christos <td>HST</td> 602 1.1 christos <td>Hawaii standard time</td> 603 1.1 christos <td>1933-04-30</td> 604 1.1 christos <td rowspan="3">02:00</td> 605 1.1 christos </tr> 606 1.1 christos <tr align="center"> 607 1.1 christos <td>−9:30</td> 608 1.1 christos <td>+1:00</td> 609 1.1 christos <td>HDT</td> 610 1.1 christos <td>Hawaii daylight time</td> 611 1.1 christos <td>1933-05-21</td> 612 1.1 christos </tr> 613 1.1 christos <tr align="center"> 614 1.1 christos <td>−10:30¹</td> 615 1.1 christos <td>−1:00¹</td> 616 1.1 christos <td>HST¹</td> 617 1.1 christos <td>Hawaii standard time</td> 618 1.1 christos <td>1942-02-09</td> 619 1.1 christos </tr> 620 1.1 christos <tr align="center"> 621 1.1 christos <td rowspan="2">−9:30</td> 622 1.1 christos <td>+1:00</td> 623 1.1 christos <td>HWT</td> 624 1.1 christos <td>Hawaii war time</td> 625 1.1 christos <td>1945-08-14</td> 626 1.1 christos <td>13:30²</td> 627 1.1 christos </tr> 628 1.1 christos <tr align="center"> 629 1.1 christos <td>0</td> 630 1.1 christos <td>HPT</td> 631 1.1 christos <td>Hawaii peace time</td> 632 1.1 christos <td>1945-09-30</td> 633 1.1 christos <td rowspan="2">02:00</td> 634 1.1 christos </tr> 635 1.1 christos <tr align="center"> 636 1.1 christos <td>−10:30</td> 637 1.1 christos <td>−1:00</td> 638 1.1 christos <td rowspan="2">HST</td> 639 1.1 christos <td rowspan="2">Hawaii standard time</td> 640 1.1 christos <td>1947-06-08</td> 641 1.1 christos </tr> 642 1.1 christos <tr align="center"> 643 1.1 christos <td>−10:00³</td> 644 1.1 christos <td>+0:30³</td> 645 1.1 christos <td colspan="2">—</td> 646 1.1 christos </tr> 647 1.1 christos <tr> 648 1.1 christos <td colspan="6"> 649 1.1 christos ¹Switching to US rules…most recent transition (in 1919) was to standard time 650 1.1 christos </td> 651 1.1 christos </tr> 652 1.1 christos <tr> 653 1.1 christos <td colspan="6"> 654 1.1 christos ²23:00 <a href="https://en.wikipedia.org/wiki/Universal_Time">UT</a> 655 1.1 christos + (−9:30) = 13:30 local 656 1.1 christos </td> 657 1.1 christos </tr> 658 1.1 christos <tr> 659 1.1 christos <td colspan="6"> 660 1.1 christos ³Since <a href="https://en.wikipedia.org/wiki/ISO_8601">1947–06–08T12:30Z</a>, 661 1.1 christos the civil time in Hawaii has been 662 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> 663 1.1 christos − 10:00 year-round. 664 1.1 christos </td> 665 1.1 christos </tr> 666 1.1 christos </table> 667 1.1 christos 668 1.1 christos <p>There will be a short quiz later. <code>8-)</code></p> 669 1.1 christos 670 1.1 christos <hr> 671 1.1 christos <address> 672 1.1 christos This web page is in the public domain, so clarified as of 673 1.1 christos 2015-10-20 by Bill Seymour. 674 1.1 christos <br> 675 1.1 christos All suggestions and corrections will be welcome; all flames will be amusing. 676 1.1 christos Mail to was at pobox dot com. 677 1.1 christos </address> 678 1.1 christos </body> 679 1.1 christos </html> 680
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