Sergei.Haller@math.uni-giessen.de(Sergei Haller) 01.06.05 17:16
no. time_t is the number of seconds since 1970-01-01 in UTC. a time value as time_t is _unique_.
the call date -d "1970-01-01 1117634400 sec"
is _not_ translation from time_t to human readable time. you are kind of "cheating" telling date to recalculate the time "1970-01-01 00:00:1117634400" in whatever time zone it expectes it.
That "cheat" is documented in some man pages for (GNU) "date".
"info date" says:
* To convert a date string to the number of seconds since the epoch (which is 1970-01-01 00:00:00 UTC), use the `--date' option with the `%s' format. That can be useful in sorting and/or graphing and/or comparing data by date. The following command outputs the number of the seconds since the epoch for the time two minutes after the epoch:
date --date='1970-01-01 00:02:00 +0000' +%s 120
If you do not specify time zone information in the date string, `date' uses your computer's idea of the time zone when interpreting the string. For example, if your computer's time zone is that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000 seconds) behind UTC:
# local time zone used date --date='1970-01-01 00:02:00' +%s 18120
* If you're sorting or graphing dated data, your raw date values may be represented as seconds since the epoch. But few people can look at the date `946684800' and casually note "Oh, that's the first second of the year 2000 in Greenwich, England."
date --date='2000-01-01 UTC' +%s 946684800
An alternative is to use the `--utc' (`-u') option. Then you may omit `UTC' from the date string. Although this produces the same result for `%s' and many other format sequences, with a time zone offset different from zero, it would give a different result for zone-dependent formats like `%z'.
date -u --date=2000-01-01 +%s 946684800
*To convert such an unwieldy number of seconds back to a more* *readable form, use a command like this:*
# local time zone used date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z" 1999-12-31 19:00:00 -0500
Often it is better to output UTC-relative date and time:
date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z" 2000-01-01 00:00:00 +0000
I don't see why this is a "cheat" (= not intended way of usesage) and why it should be required to have "self programming" such function. Of cause the risc of errors is low if all use exactly teh same routine.
The man page example is missleading. date will interpret the "1970-01-01" with the current timezone...
It must read: # date -d "1970-01-01 UTC 1117634400 sec" Wed Jun 1 16:00:00 CEST 2005 as the "-utc" relates to output. # date -u -d "1970-01-01 UTC 1117634400 sec" Wed Jun 1 14:00:00 UTC 2005
But i still do not undestand why here is a 1h offset if nothing is given. I would assuem that the timezone would be komensated # date -d "1970-01-01 1117634400 sec" Wed Jun 1 15:00:00 CEST 2005
BTW: # date -u -d "1970-01-01 1117634400 sec " Wed Jun 1 14:00:00 UTC 2005
# date -u -d "1970-01-01 1117634400 sec GMT" Wed Jun 1 14:00:00 UTC 2005
# date -d "1970-01-01 1117634400 sec GMT" Wed Jun 1 16:00:00 CEST 2005
# date -u -d "1970-01-01 1117634400 sec CEST" Wed Jun 1 12:00:00 UTC 2005 # date -u -d "1970-01-01 1117634400 sec CET" Wed Jun 1 13:00:00 UTC 2005 Two different times...
# date -u -d "1970-01-01 1117634400 sec MET" Wed Jun 1 13:00:00 UTC 2005 # date -u -d "1970-01-01 1117634400 sec MEST" Wed Jun 1 12:00:00 UTC 2005 Two different times...
# date -d "1970-01-01 1117634400 sec CET" Wed Jun 1 15:00:00 CEST 2005 # date -d "1970-01-01 1117634400 sec CEST" Wed Jun 1 15:00:00 CEST 2005 One time ...
# date -u -d "1970-01-01 1117634400 sec" +"%D %T %z %Z" 06/01/05 14:00:00 +0000 UTC # date -u -d "1970-01-01 UTC 1117634400 sec" +"%D %T %z %Z" 06/01/05 14:00:00 +0000 UTC # date -u -d "1970-01-01 MET 1117634400 sec" +"%D %T %z %Z" 06/01/05 13:00:00 +0000 UTC # date -u -d "1970-01-01 MEST 1117634400 sec" +"%D %T %z %Z" 06/01/05 12:00:00 +0000 UTC # date -d "1970-01-01 1117634400 sec" +"%D %T %z %Z" 06/01/05 15:00:00 +0200 CEST # date -d "1970-01-01 UTC 1117634400 sec" +"%D %T %z %Z" 06/01/05 16:00:00 +0200 CEST
# date -d "1970-01-01 MET 1117634400 sec" +"%D %T %z %Z" 06/01/05 15:00:00 +0200 CEST # date -d "1970-01-01 MEST 1117634400 sec" +"%D %T %z %Z" 06/01/05 14:00:00 +0200 CEST
# date -d "1970-01-01 1117634400 sec CEST" +"%D %T %z %Z" 06/01/05 15:00:00 +0200 CEST # date -d "1970-01-01 1117634400 sec CET" +"%D %T %z %Z" 06/01/05 15:00:00 +0200 CEST
That looks like "MEST" is defined other than "CEST"?
Rainer