1.\" $NetBSD: zic.8,v 1.39 2022/08/24 06:39:14 christos Exp $ 2.Dd August 24, 2022 3.Dt ZIC 8 4.Os 5.Sh NAME 6.Nm zic 7.Nd timezone compiler 8.Sh SYNOPSIS 9.Nm 10.Op Fl \-version 11.Op Fl \-help 12.Op Fl b 13.Op Fl d Ar directory 14.Op Fl L Ar leapsecondfilename 15.Op Fl l Ar localtime 16.Op Fl p Ar posixrules 17.Op Fl s 18.Op Fl t Ar file 19.Op Fl v 20.Op Fl y Ar command 21.Op Ar Filename ... 22.Sh DESCRIPTION 23The 24.Nm 25program reads text from the file(s) named on the command line 26and creates the timezone information format (TZif) files 27specified in this input. 28If a 29.Ar filename 30is 31.Ar \&- , 32standard input is read. 33.Pp 34.Sh OPTIONS 35.Bl -tag -width XXXXXXXXXX -compact 36.It Fl \-version 37Output version information and exit. 38.It Fl \-help 39Output short usage message and exit. 40.It Fl b Ar bloat 41Output backward-compatibility data as specified by 42.Ar bloat . 43If 44.Ar bloat 45is 46.Dv fat , 47generate additional data entries that work around potential bugs or 48incompatibilities in older software, such as software that mishandles 49the 64-bit generated data. 50If 51.Ar bloat 52is 53.Dv slim , 54keep the output files small; this can help check for the bugs 55and incompatibilities. 56The default is 57.Dv slim , 58as software that mishandles 64-bit data typically 59mishandles timestamps after the year 2038 anyway. 60Also see the 61.Fl r 62option for another way to alter output size. 63.It Fl d Ar directory 64Create time conversion information files in the named directory rather than 65in the standard directory named below. 66.It Fl l Ar timezone 67Use the 68.Ar timezone 69as local time. 70.Nm 71will act as if the input contained a link line of the form 72.Dl Link timezone localtime 73If 74.Ar timezone 75is 76.Dv \&- , 77any already-existing link is removed. 78.It Fl L Ar leapsecondfilename 79Read leap second information from the file with the given name. 80If this option is not used, 81no leap second information appears in output files. 82.It Fl p Ar timezone 83Use 84.Ar timezone's 85rules when handling POSIX-format 86TZ strings like 87.Qq EET\&-2EEST 88that lack transition rules. 89.Nm 90will act as if the input contained a link line of the form 91.Dl Link timezone posixrules 92.Pp 93This feature is obsolete and poorly supported. 94Among other things it should not be used for timestamps after the year 2037, 95and it should not be combined with 96.Fl b Ar slim 97if 98.Va timezone's 99transitions are at standard time or Universal Time (UT) instead of local time. 100If 101.Ar timezone 102is 103.Dv \&- , 104any already-existing link is removed. 105.It Fl r Op Ar @lo / Op Ar @hi 106Limit the applicability of output files 107to timestamps in the range from 108.Ar lo 109(inclusive) to 110.Ar hi 111(exclusive), where 112.Ar lo 113and 114.Ar hi 115are possibly-signed decimal counts of seconds since the Epoch 116(1970-01-01 00:00:00 UTC). 117Omitted counts default to extreme values. 118The output files use UT offset 0 and abbreviation 119.q "\*-00" 120in place of the omitted timestamp data. 121For example, 122.Bd -literal 123zic -r @0 124.Ed 125omits data intended for negative timestamps (i.e., before the Epoch), and 126.Bd -literal 127zic -r @0/@2147483648 128.Ed 129outputs data intended only for nonnegative timestamps that fit into 13031-bit signed integers. 131Or using 132.Xr date 1 , 133.Bd -literal 134zic -r @$(date +%s) 135.Ed 136omits data intended for past timestamps. 137Although this option typically reduces the output file's size, 138the size can increase due to the need to represent the timestamp range 139boundaries, particularly if 140.Ar hi 141causes a TZif file to contain explicit entries for 142.Ar pre- hi 143transitions rather than concisely representing them 144with an extended POSIX TZ string. 145Also see the 146.Fl b Ar slim 147option for another way to shrink output size. 148.It Fl R Ar @hi 149Generate redundant trailing explicit transitions for timestamps 150that occur less than 151Ar hi 152seconds since the Epoch, even though the transitions could be 153more concisely represented via the extended POSIX TZ string. 154This option does not affect the represented timestamps. 155Although it accommodates nonstandard TZif readers 156that ignore the extended POSIX TZ string, 157it increases the size of the altered output files. 158.It Fl t Ar file 159When creating local time information, put the configuration link in 160the named file rather than in the standard location. 161.It Fl v 162Be more verbose, and complain about the following situations: 163.Bl -dash 164.It 165The input specifies a link to a link. 166.It 167A year that appears in a data file is outside the range 168of representable years. 169.It 170A time of 24:00 or more appears in the input. 171Pre-1998 versions of 172.Nm 173prohibit 24:00, and pre-2007 versions prohibit times greater than 24:00. 174.It 175A rule goes past the start or end of the month. 176Pre-2004 versions of 177.Nm 178prohibit this. 179.It 180A time zone abbreviation uses a 181.Dv %z 182format. 183Pre-2015 versions of 184.Nm 185do not support this. 186.It 187A timestamp contains fractional seconds. 188Pre-2018 versions of 189.Nm 190do not support this. 191.It 192The input contains abbreviations that are mishandled by pre-2018 versions of 193.Nm 194due to a longstanding coding bug. 195These abbreviations include 196.Qq L 197for 198.Qq Link , 199.Qq mi 200for 201.Qq min , 202.Qq Sa 203for 204.Qq Sat , 205and 206.Qq Su 207for 208.Qq Sun . 209.It 210The output file does not contain all the information about the 211long-term future of a timezone, because the future cannot be summarized as 212an extended POSIX TZ string. 213For example, as of 2019 this problem 214occurs for Iran's daylight-saving rules for the predicted future, as 215these rules are based on the Iranian calendar, which cannot be 216represented. 217.It 218The output contains data that may not be handled properly by client 219code designed for older 220.Xr zic 8 221output formats. 222These compatibility issues affect only timestamps 223before 1970 or after the start of 2038. 224.It 225The output contains a truncated leap second table, 226which can cause some older TZif readers to misbehave. 227This can occur if the 228.Fl L 229option is used, and either an Expires line is present or 230the 231.Fl r 232option is also used. 233.It 234The output file contains more than 1200 transitions, 235which may be mishandled by some clients. 236The current reference client supports at most 2000 transitions; 237pre-2014 versions of the reference client support at most 1200 238transitions. 239.It 240A time zone abbreviation has fewer than 3 or more than 6 characters. 241POSIX requires at least 3, and requires implementations to support 242at least 6. 243.It 244An output file name contains a byte that is not an ASCII letter, 245.Qq - , 246.Qq / , 247or 248.Qq _ ; 249or it 250or it contains a file name component that contains more than 14 bytes 251or that starts with 252.Qq - . 253.El 254.El 255.Pp 256Input files should be text files, that is, they should be a series of 257zero or more lines, each ending in a newline byte and containing at 258most 2048 bytes counting the newline, and without any 259.Dv NUL 260bytes. 261The input text's encoding 262is typically UTF-8 or ASCII; it should have a unibyte representation 263for the POSIX Portable Character Set (PPCS) 264.Rs 265.%U https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap06.html 266.Re 267and the encoding's non-unibyte characters should consist entirely of 268non-PPCS bytes. 269Non-PPCS characters typically occur only in comments: 270although output file names and time zone abbreviations can contain 271nearly any character, other software will work better if these are 272limited to the restricted syntax described under the 273.Op v 274option. 275.Pp 276Input lines are made up of fields. 277Fields are separated from one another by one or more white space characters. 278The white space characters are space, form feed, carriage return, newline, 279tab, and vertical tab. 280Leading and trailing white space on input lines is ignored. 281An unquoted sharp character (#) in the input introduces a comment which extends 282to the end of the line the sharp character appears on. 283White space characters and sharp characters may be enclosed in double 284quotes 285.Pq \&" 286.\" XXX " 287if they're to be used as part of a field. 288Any line that is blank (after comment stripping) is ignored. 289Nonblank lines are expected to be of one of three types: 290rule lines, zone lines, and link lines. 291.Pp 292Names must be in English and are case insensitive. 293They appear in several contexts, and include month and weekday names 294and keywords such as 295.Qq maximum , 296.Qq only , 297.Qq Rolling , 298and 299.Qq Zone . 300A name can be abbreviated by omitting all but an initial prefix; any 301abbreviation must be unambiguous in context. 302.Pp 303A rule line has the form 304.Pp 305.Dl Rule NAME FROM TO \&- IN ON AT SAVE LETTER/S 306.Pp 307For example: 308.Pp 309.Dl Rule US 1967 1973 \&- Apr lastSun 2:00w 1:00d D 310.Pp 311The fields that make up a rule line are: 312.Bl -tag -width "LETTER/S" 313.It NAME 314Gives the name of the rule set that contains this line. 315The name must start with a character that is neither 316an ASCII digit nor 317.Ar \&- 318nor 319.Ar + . 320To allow for future extensions, 321an unquoted name should not contain characters from the set 322.Ar !$%&'()*,/:;<=>?@[\e]^`{|}~ . 323.It FROM 324Gives the first year in which the rule applies. 325Any signed integer year can be supplied; the proleptic Gregorian calendar 326is assumed, with year 0 preceding year 1. 327The word 328.Em minimum 329(or an abbreviation) means the indefinite past. 330The word 331.Em maximum 332(or an abbreviation) means the indefinite future. 333Rules can describe times that are not representable as time values, 334with the unrepresentable times ignored; this allows rules to be portable 335among hosts with differing time value types. 336.It TO 337Gives the final year in which the rule applies. 338In addition to 339.Em minimum 340and 341.Em maximum 342(as above), 343the word 344.Em only 345(or an abbreviation) 346may be used to repeat the value of the 347.Em FROM 348field. 349.It \&- 350should be 351.Qq \&- 352for compatibility with older versions of 353.Nm . 354It was previously known as the 355.Em TYPE 356field, which could contain values to allow a 357separate script to further restrict in which 358.Em types 359of years the rule would apply. 360.It IN 361Names the month in which the rule takes effect. 362Month names may be abbreviated. 363.It ON 364Gives the day on which the rule takes effect. 365Recognized forms include: 366.Pp 367.Bl -tag -width lastSun -compact -offset indent 368.It 5 369the fifth of the month 370.It lastSun 371the last Sunday in the month 372.It lastMon 373the last Monday in the month 374.It Sun\*[Ge]8 375first Sunday on or after the eighth 376.It Sun\*[Le]25 377last Sunday on or before the 25th 378.El 379.Pp 380Names of days of the week may be abbreviated or spelled out in full. 381A weekday name (e.g., 382.Qq Sunday ) 383or a weekday name preceded by 384.Qq last 385(e.g., 386.Qq lastSunday ) 387may be abbreviated or spelled out in full. 388There must be no white space characters within the 389.Em ON 390field. 391The 392.Qq <= 393and 394.Qq >= 395constructs can result in a day in the neighboring month; 396for example, the IN-ON combination 397.Qq "Oct Sun>=31" 398tands for the first Sunday on or after October 31, 399even if that Sunday occurs in November. 400.It AT 401Gives the time of day at which the rule takes effect, 402relative to 00:00, the start of a calendar day. 403Recognized forms include: 404.Pp 405.Bl -tag -width "00X19X32X13" -compact -offset indent 406.It 2 407time in hours 408.It 2:00 409time in hours and minutes 410.It 01:28:14 411time in hours, minutes, and seconds 412.It 00:19:32.13 413time with fractional seconds 414.It 12:00 415midday, 12 hours after 00:00 416.It 15:00 4173 PM, 15 hours after 00:00 418.It 24:00 419end of day, 24 hours after 00:00 420.It 260:00 421260 hours after 00:00 422.It \-2:30 4232.5 hours before 00:00 424.It \- 425equivalent to 0 426.El 427.Pp 428Although 429.I zic 430rounds times to the nearest integer second 431(breaking ties to the even integer), the fractions may be useful 432to other applications requiring greater precision. 433The source format does not specify any maximum precision. 434Any of these forms may be followed by the letter 435.Em w 436if the given time is local or 437.Qq wall clock 438time, 439.Em s 440if the given time is standard time without any adjustment for daylight saving, 441or 442.Em u 443(or 444.Em g 445or 446.Em z ) 447if the given time is universal time; 448in the absence of an indicator, 449local (wall clock) time is assumed. 450These forms ignore leap seconds; for example, 451if a leap second occurs at 00:59:60 local time, 452.q "1:00" 453stands for 3601 seconds after local midnight instead of the usual 3600 seconds. 454The intent is that a rule line describes the instants when a 455clock/calendar set to the type of time specified in the 456.Em AT 457field would show the specified date and time of day. 458.It SAVE 459Gives the amount of time to be added to local standard time when the rule is in 460effect, and whether the resulting time is standard or daylight saving. 461This field has the same format as the 462.Em AT 463field 464.Em s 465for standard time and 466.Em d 467for daylight saving time. 468The suffix letter is typically omitted, and defaults to 469.Em s 470if the offset is zero and to 471.Em d 472otherwise. 473Negative offsets are allowed; in Ireland, for example, daylight saving 474time is observed in winter and has a negative offset relative to 475Irish Standard Time. 476The offset is merely added to standard time; for example, 477.Nm 478does not distinguish a 10:30 standard time plus an 0:30 479.Em SAVE 480from a 10:00 standard time plus a 1:00 481.Em SAVE . 482.It LETTER/S 483Gives the 484.Qq variable part 485(for example, the 486.Qq S 487or 488.Qq D 489in 490.Qq EST 491or 492.Qq EDT ) 493of time zone abbreviations to be used when this rule is in effect. 494If this field is 495.Em \&- , 496the variable part is null. 497.El 498.Pp 499A zone line has the form 500.Pp 501.Dl Zone NAME STDOFF RULES/SAVE FORMAT [UNTIL] 502.Pp 503For example: 504.Pp 505.Dl Zone Asia/Amman 2:00 Jordan EE%sT 2017 Oct 27 1:00 506.Pp 507The fields that make up a zone line are: 508.Bl -tag -width "RULES/SAVE" 509.It NAME 510The name of the timezone. 511This is the name used in creating the time conversion information file for the 512timezone. 513It should not contain a file name component 514.Qq . 515or 516.Qq .. ; 517a file name component is a maximal substring that does not contain 518.Qq / . 519.It STDOFF 520The amount of time to add to UT to get standard time, 521without any adjustment for daylight saving. 522This field has the same format as the 523.Em AT 524and 525.Em SAVE 526fields of rule lines, except without suffix letters; 527begin the field with a minus sign if time must be subtracted from UT. 528.It RULES 529The name of the rules that apply in the timezone or, 530alternatively, a field in the same format as a rule-line SAVE column, 531giving the amount of time to be added to local standard time 532and whether the resulting time is standard or daylight saving. 533If this field is 534.Em \&- 535then standard time always applies. 536When an amount of time is given, only the sum of standard time and 537this amount matters. 538.It FORMAT 539The format for time zone abbreviations. 540The pair of characters 541.Em %s 542is used to show where the 543.Qq variable part 544of the time zone abbreviation goes. 545Alternatively, a format can use the pair of characters 546.Em %z 547+to stand for the UT offset in the form 548.Em \(+- hh , 549.Em \(+- hhmm , 550or 551.Em \(+- hhmmss , 552using the shortest form that does not lose information, where 553.Em hh , 554.Em mm , 555and 556.Em ss 557are the hours, minutes, and seconds east (+) or west (\-) of UT. 558Alternatively, 559a slash 560.Pq \&/ 561separates standard and daylight abbreviations. 562To conform to POSIX, a time zone abbreviation should contain only 563alphanumeric ASCII characters, 564.Qq + 565and 566.Qq \&- . 567By convention, the time zone abbreviation 568.Qq \&-00 569is a placeholder that means local time is unspecified. 570.It UNTIL 571The time at which the UT offset or the rule(s) change for a location. 572It takes the form of one to four fields YEAR [MONTH [DAY [TIME]]]. 573If this is specified, 574the time zone information is generated from the given UT offset 575and rule change until the time specified, which is interpreted using 576the rules in effect just before the transition. 577The month, day, and time of day have the same format as the IN, ON, and AT 578fields of a rule; trailing fields can be omitted, and default to the 579earliest possible value for the missing fields. 580.Pp 581The next line must be a 582.Qq continuation 583line; this has the same form as a zone line except that the 584string 585.Qq Zone 586and the name are omitted, as the continuation line will 587place information starting at the time specified as the 588.Em until 589information in the previous line in the file used by the previous line. 590Continuation lines may contain 591.Em until 592information, just as zone lines do, indicating that the next line is a further 593continuation. 594.El 595.Pp 596If a zone changes at the same instant that a rule would otherwise take 597effect in the earlier zone or continuation line, the rule is ignored. 598A zone or continuation line 599.I L 600with a named rule set starts with standard time by default: 601that is, any of 602.IR L 's 603timestamps preceding 604.IR L 's 605earliest rule use the rule in effect after 606.IR L 's 607first transition into standard time. 608In a single zone it is an error if two rules take effect at the same 609instant, or if two zone changes take effect at the same instant. 610.Pp 611If a continuation line subtracts 612.Dv N 613seconds from the UT offset after a transition that would be 614interpreted to be later if using the continuation line's UT offset and 615rules, the 616.Em until 617time of the previous zone or continuation line is interpreted 618according to the continuation line's UT offset and rules, and any rule 619that would otherwise take effect in the next 620.Dv N 621seconds is instead assumed to take effect simultaneously. 622For example: 623.Pp 624.Bl -column -compact -offset indent "# Rule" "Swiss" "FROM" "1995" "\&*" "Oct" "lastSun" "1:00u" "SAVE" "LETTER/S" 625.It # Rule NAME FROM TO \&- IN ON AT SAVE LETTER/S 626.It Rule US 1967 2006 \&- Oct lastSun 2:00 0 S 627.It Rule US 1967 1973 \&- Apr lastSun 2:00 1:00 D 628.It # Zone NAME STDOFF RULES FORMAT [UNTIL] 629.It Zone America/Menominee \&-5:00 \&- EST 1973 Apr 29 2:00 630.It \&-6:00 US C%sT 631.El 632.Pp 633Here, an incorrect reading would be there were two clock changes on 1973-04-29, 634the first from 02:00 EST (\&-05) to 01:00 CST (\&-06), 635and the second an hour later from 02:00 CST (\&-06) to 03:00 CDT (\&-05). 636However, 637.Nm 638interprets this more sensibly as a single transition from 02:00 CST (\&-05) to 63902:00 CDT (\&-05). 640.Pp 641A link line has the form 642.Pp 643.Dl Link TARGET LINK-NAME 644.Pp 645For example: 646.Pp 647.Dl Link Europe/Istanbul Asia/Istanbul 648.Pp 649The 650.Em TARGET 651field should appear as the 652.Em NAME 653field in some zone line. 654The 655.Em LINK-NAME 656field is used as an alternative name for that zone; 657it has the same syntax as a zone line's 658.Em NAME 659field. 660.Pp 661Except for continuation lines, 662lines may appear in any order in the input. 663However, the behavior is unspecified if multiple zone or link lines 664define the same name, or if the source of one link line is the target 665of another. 666.Pp 667The file that describes leap seconds can have leap lines and an 668expiration line. 669Leap lines have the following form: 670.Pp 671.Dl Leap YEAR MONTH DAY HH:MM:SS CORR R/S 672.Pp 673For example: 674.Pp 675.Dl Leap 2016 Dec 31 23:59:60 + S 676.Pp 677The 678.Em YEAR , 679.Em MONTH , 680.Em DAY , 681and 682.Em HH:MM:SS 683fields tell when the leap second happened. 684The 685.Em CORR 686field 687should be 688.Qq \&+ 689if a second was added 690or 691.Qq \&- 692if a second was skipped. 693The 694.Em R/S 695field 696should be (an abbreviation of) 697.Qq Stationary 698if the leap second time given by the other fields should be interpreted as UTC 699or 700(an abbreviation of) 701.Qq Rolling 702if the leap second time given by the other fields should be interpreted as 703local (wall clock) time. 704.Pp 705Rolling leap seconds were implemented back when it was not 706clear whether common practice was rolling or stationary, 707with concerns that one would see 708Times Square ball drops where there'd be a 709.Qq 3... 2... 1... leap... Happy New Year 710countdown, placing the leap second at 711midnight New York time rather than midnight UTC. 712However, this countdown style does not seem to have caught on, 713which means rolling leap seconds are not used in practice; 714also, they are not supported if the 715.Fl r 716option is used. 717.Pp 718The expiration line, if present, has the form: 719.Pp 720.Dl Expires YEAR MONTH DAY HH:MM:SS 721.Pp 722For example: 723.Pp 724.Dl Expires 2020 Dec 28 00:00:00 725.Pp 726The 727.Em YEAR , 728.Em MONTH , 729.Em DAY , 730and 731.Em HH:MM:SS 732fields give the expiration timestamp in UTC for the leap second table. 733.Sh EXTENDED EXAMPLE 734Here is an extended example of 735.Ic zic 736input, intended to illustrate many of its features. 737.Pp 738.Bl -column -compact -offset indent "# Rule" "Swiss" "FROM" "1995" "\&*" "Oct" "lastSun" "1:00u" "SAVE" "LETTER/S" 739.It # Rule NAME FROM TO \&- IN ON AT SAVE LETTER/S 740.It Rule Swiss 1941 1942 \&- May Mon>=1 1:00 1:00 S 741.It Rule Swiss 1941 1942 \&- Oct Mon>=1 2:00 0 - 742.Pp 743.It Rule EU 1977 1980 \&- Apr Sun>=1 1:00u 1:00 S 744.It Rule EU 1977 only \&- Sep lastSun 1:00u 0 - 745.It Rule EU 1978 only \&- Oct 1 1:00u 0 - 746.It Rule EU 1979 1995 \&- Sep lastSun 1:00u 0 - 747.It Rule EU 1981 max \&- Mar lastSun 1:00u 1:00 S 748.It Rule EU 1996 max \&- Oct lastSun 1:00u 0 - 749.El 750.Pp 751.Bl -column -compact -offset indent "# Zone" "Europe/Zurich" "0:29:45.50" "RULES/SAVE" "FORMAT" "UNTIL" 752.It # Zone NAME STDOFF RULES/SAVE FORMAT [UNTIL] 753.It Zone Europe/Zurich 0:34:08 \&- LMT 1853 Jul 16 754.It 0:29:45.50 \&- BMT 1894 Jun 755.It 1:00 Swiss CE%sT 1981 756.It 1:00 EU CE%sT 757.Pp 758.El 759.Bl -column -compact -offset indent "# Zone" "Europe/Zurich" "0:34:08" "RULES/SAVE" "FORMAT" "UNTIL" 760.It Link Europe/Zurich Europe/Vaduz 761.El 762.Pp 763In this example, the EU rules are for the European Union 764and for its predecessor organization, the European Communities. 765The timezone is named Europe/Zurich and it has the alias Europe/Vaduz. 766This example says that Zurich was 34 minutes and 8 767seconds east of UT until 1853-07-16 at 00:00, when the legal offset 768was changed to 7\(de\|26\(fm\|22.50\(sd; which this works out to 7690:29:45.50; 770.Nm 771treats this by rounding it to 0:29:46. 772After 1894-06-01 at 00:00 the UT offset became one hour 773and Swiss daylight saving rules (defined with lines beginning with 774.Qq "Rule Swiss" 775apply. 776From 1981 to the present, EU daylight saving rules have 777From 1981 to the present, EU daylight saving rules have 778.Pp 779In 1941 and 1942, daylight saving time applied from the first Monday 780in May at 01:00 to the first Monday in October at 02:00. 781The pre-1981 EU daylight-saving rules have no effect 782here, but are included for completeness. 783Since 1981, daylight 784saving has begun on the last Sunday in March at 01:00 UTC. 785Until 1995 it ended the last Sunday in September at 01:00 UTC, 786but this changed to the last Sunday in October starting in 1996. 787.Pp 788For purposes of display, 789.Qq LMT 790and 791.Qq BMT 792were initially used, respectively. 793Since 794Swiss rules and later EU rules were applied, the time zone abbreviation 795has been CET for standard time and CEST for daylight saving 796time. 797.Sh FILES 798Input files use the format described in this section; output files use 799.Xr tzfile 5 800format. 801.Bl -tag -width /usr/share/zoneinfo -compact 802.It Pa /etc/localtime 803Default local timezone file 804.It Pa /usr/share/zoneinfo 805Default timezone information directory 806.El 807.Sh NOTES 808For areas with more than two types of local time, 809you may need to use local standard time in the 810.Em AT 811field of the earliest transition time's rule to ensure that 812the earliest transition time recorded in the compiled file is correct. 813.Pp 814If, 815for a particular timezone, 816a clock advance caused by the start of daylight saving 817coincides with and is equal to 818a clock retreat caused by a change in UT offset, 819.Ic zic 820produces a single transition to daylight saving at the new UT offset 821without any change in local (wall clock) time. 822To get separate transitions 823use multiple zone continuation lines 824specifying transition instants using universal time. 825.Sh SEE ALSO 826.Xr tzfile 5 , 827.Xr zdump 8 828.\" @(#)zic.8 8.6 829.\" This file is in the public domain, so clarified as of 830.\" 2009-05-17 by Arthur David Olson. 831