timezone

setting local time zone

Miscellaneous Information

SYNOPSIS

TZ=standardHH[:MM[:SS]] [daylight[HH[:MM[:SS]]] [,startdate[/starttime], enddate[/endtime]]]

DESCRIPTION

The time kept by the local machine should be a universal standard representation such as Greenwich Mean Time (GMT) or Universal Time Coordinated (UTC) -- hereafter referred to as the universal reference time. For personal computers that are not sharing data across time zones, the local time is an adequate standard. To support a universal standard, all MKS utilities assume that times stored in the file system and returned by the operating system are stored in the universal reference time, then translated to local times. The mapping from the universal reference time to local time is specified by the TZ (time zone) environment variable. If left undefined, the TZ variable defaults to the current time zone setting of your operating system.

The value of the TZ variable has the following five fields
— two required and three optional:
standard

An alphabetic abbreviation for the local standard time zone (for example, GMT, EST, MSEZ).

HH[:MM[:SS]]

The time offset westwards from the universal reference time. A leading minus sign (-) means that the local time zone is east of the universal reference time. An offset of this form must follow standard and can also optionally follow daylight. An optional colon (:) separates hours from optional minutes and seconds.

If daylight is specified without a daylight offset, daylight savings time is assumed to be one hour ahead of the standard time.

[daylight]

The abbreviation for your local daylight savings time zone. If the first and third fields are identical or third field is missing, Daylight Saving Time conversion is disabled. The number of hours, minutes, and seconds your local Daylight Savings Time is offset from UTC when Daylight Savings Time is in effect. If the Daylight Savings Time abbreviation is specified, and the offset omitted, the offset of one hour is assumed.

[,startdate[/starttime],enddate[/endtime]]

A rule that identifies the start and end of Daylight Savings Time -- specifying when Daylight Savings Time should be in effect. Both the startdate and enddate must be present, and must either take the form Jn, n, or Mm.n.d..

  • Jn is the Julian day n (1 <= n <= 365) and does not account for leap days.
  • n is the zero-based Julian day (0 <= n <= 365). Leap days are counted; therefore, you may refer to February 29th.
  • For Mm.n.d, the dth day (0 <= d <= 6) of week n of month m of the year (1 <= n <= 5, 1 <= m <= 12 where week 5 is the last d day in month m, which may occur in either the fourth or fifth week). In addition, week 1 is the first week where the dth day occurs, and day zero is Sunday.

Neither starttime nor endtime are required, and when omitted, their values default to 02:00:00. If this Daylight Savings Time rule is omitted altogether, the values in the rule default to the standard American Daylight Savings Time rules -- starting at 02:00:00 the first Sunday in April and ending at 02:00:00 the last Sunday in October.

EXAMPLES

Here are some possible settings for the North American Eastern time zone: 
TZ=EST5EDT
TZ=EST0EDT
TZ=EST0
In the first case, the reference time is GMT and thus stored time values are correct world wide. A simple change of the TZ variable prints local time correctly, anywhere. In the second case, the reference time is Eastern Standard Time and the only conversion performed is for Daylight Saving Time. Therefore, there is no need to adjust the hardware clock for Daylight Saving Time twice per year. In the third case, the reference time is always the time reported. This is suggested if the hardware clock on your machine automatically adjusts for Daylight Saving Time or you insist on manually resetting the hardware time twice a year.

Other examples include: 
TZ=NST3:30NDT2:00
TZ=MSEZ-1
The first applies to Newfoundland, while the second works in most of Western Europe.

Here are some time zone scenarios that involve Daylight Savings Time specification: 
TZ=PST0PDT-1
TZ=ACST-09:30ACDT-10:30,M10.5.0/2:00,M3.5.0/2:00
The first scenario shows the TZ of a person in Seattle who stores local time on a PC, but does not adjust the clock to agree with Daylight Savings Time. The stated time zone precedes the machine clock time by one hour when Daylight Savings Time is in effect.

The second scenario shows the TZ set by a person in Australia who sets a PC clock to UTC and never adjusts it. The machine clock precedes UTC by 9.5 hours when Daylight Savings Time is not in effect, and by 10.5 hours when in effect. Daylight Savings Time is in effect from 2:00 am on the last Sunday in October until 2:00 am on the last Sunday in March.

PORTABILITY

This interpretation of the TZ variable is a superset of that supported by UNIX System V.

SEE ALSO

Commands:
date, touch

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