Daylight Savings and UNIX?

Anonymous asks: "My company recently asked me to write them a report on how UNIX properly handles the switch to Daylight Savings Time, and back again. When our systems administrators received the report, I was somewhat surprised. Many of them weren't aware that 'cron' would run the affected jobs twice in the fall, and not at all in the spring. Apparently, the man pages on some operating systems, like Solaris, aren't forthcoming with details. Others groups, like database administrators, are completely unaware of the differences between epoch time and wall clock time. Are even technical users ignorant on how UNIX handles time, time zones, and time conversion?"

Comments

Wrong

[gus goose]

This is taken from the HP Cron man page.

Spring and Autumn Time Transitions
On the days of daylight savings (summer) time transition (in time zones and countries where daylight savings time applies), cron schedules commands differently from normal. In the following description, an ambiguous time refers to an hour and minute that occurs twice in the same day because of a daylight savings time transition (usually on a day during the Autumn season). A nonexistent time refers to an hour and minute that does not occur because of a daylight savings time transition (usually on a day during the Spring season). DST-shift refers to the offset that is applied to standard time to result in daylight savings time. This is normally one hour, but can be any combination of hours and minutes up to 23 hours and 59 minutes (see tztab(4)). When a command is specified to run at an ambiguous time, the command is executed only once at the first occurrence of the ambiguous time. When a command is specified to run at a nonexistent time, the command is executed after the specified time by an amount of time equal to the DST-shift. When such an adjustment would conflict with another time specified to run the command, the command is run only once rather than running the command twice at the same time. Commands that are scheduled to run during all hours (there is a * is in the hour field of the crontab entry) are scheduled without any adjustment.

This is taken from the Linux cron manpage though:

Note that this means that non-existant times, such as "missing hours" during daylight savings conversion, will never match, causing jobs scheduled during the "missing times" not to be run. Similarly, times that occur more than once (again, during daylight savings conversion) will cause matching jobs to be run twice.

So, if you want things to work, use HP-Unix.

gus

P.S. Get to it, linux guys ... maybe I will have a look.

Nope

[Clue4All]

Are even technical users ignorant on how UNIX handles time, time zones, and time conversion?

Nope, technical users know that UNIX systems should be run using GMT, which doesn't observe daylight savings time.

Re:Nope

[Covener]

Technical users know UNIX systems should keep their hardware clocks in UTC, but should obviously be aware of timezones and local time.