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Convert a time value from one timezone to another in a portable way
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Ian Zimmerman
Oct 31, 2017, 1:49:16 AM10/31/17
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I'll be satisfied if it is portable between GNU/Linux and FreeBSD.
Both systems' date(1) command provides a way to format and print an
arbitrary time, not just the current time. But unfortunately each of
them requires very different flags for date(1) to do that.
Here's what I do now on GNU/Linux (simplified):
$ date -d 'TZ="Europe/Prague" 2017-10-31 15:45' +'%H%M %m/%d/%Y'
0745 10/31/2017
But on FreeBSD, the -d option does something completely different, and
instead one is apparently supposed to just pass a bare time value (as if
setting a new system time), and include the -j flag (which OTOH is
absent in the GNU version).
If there's no other way, I'll switch on the output of uname -s, but I'd
really like to avoid that.
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Both systems' date(1) command provides a way to format and print an
arbitrary time, not just the current time. But unfortunately each of
them requires very different flags for date(1) to do that.
Here's what I do now on GNU/Linux (simplified):
$ date -d 'TZ="Europe/Prague" 2017-10-31 15:45' +'%H%M %m/%d/%Y'
0745 10/31/2017
But on FreeBSD, the -d option does something completely different, and
instead one is apparently supposed to just pass a bare time value (as if
setting a new system time), and include the -j flag (which OTOH is
absent in the GNU version).
If there's no other way, I'll switch on the output of uname -s, but I'd
really like to avoid that.
--
Please don't Cc: me privately on mailing lists and Usenet,
if you also post the followup to the list or newsgroup.
To reply privately _only_ on Usenet, fetch the TXT record for the domain.
You can automate that, Mr. Pointed Ears! :-)
Bit Twister
Oct 31, 2017, 5:52:26 AM10/31/17
to
On Mon, 30 Oct 2017 22:49:05 -0700, Ian Zimmerman wrote:
> I'll be satisfied if it is portable between GNU/Linux and FreeBSD.
>
> Both systems' date(1) command provides a way to format and print an
> arbitrary time, not just the current time. But unfortunately each of
> them requires very different flags for date(1) to do that.
>
> Here's what I do now on GNU/Linux (simplified):
>
> $ date -d 'TZ="Europe/Prague" 2017-10-31 15:45' +'%H%M %m/%d/%Y'
> 0745 10/31/2017
>
> But on FreeBSD, the -d option does something completely different,
does FreeBSD's date support the long switch option. --date?
> I'll be satisfied if it is portable between GNU/Linux and FreeBSD.
>
> Both systems' date(1) command provides a way to format and print an
> arbitrary time, not just the current time. But unfortunately each of
> them requires very different flags for date(1) to do that.
>
> Here's what I do now on GNU/Linux (simplified):
>
> $ date -d 'TZ="Europe/Prague" 2017-10-31 15:45' +'%H%M %m/%d/%Y'
> 0745 10/31/2017
>
> But on FreeBSD, the -d option does something completely different,
Janis Papanagnou
Oct 31, 2017, 10:00:28 AM10/31/17
to
Subject: Convert a time value from one timezone to another in a portable way
On 31.10.2017 06:49, Ian Zimmerman wrote:
> I'll be satisfied if it is portable between GNU/Linux and FreeBSD.
>
> Both systems' date(1) command provides a way to format and print an
> arbitrary time, not just the current time. But unfortunately each of
> them requires very different flags for date(1) to do that.
>
> Here's what I do now on GNU/Linux (simplified):
>
> $ date -d 'TZ="Europe/Prague" 2017-10-31 15:45' +'%H%M %m/%d/%Y'
> 0745 10/31/2017
>
> But on FreeBSD, the -d option does something completely different, and
> instead one is apparently supposed to just pass a bare time value (as if
> setting a new system time), and include the -j flag (which OTOH is
> absent in the GNU version).
>
> If there's no other way, I'll switch on the output of uname -s, but I'd
> really like to avoid that.
You could use another tool to do that; one possibility is GNU awk with
its time functions. But that will probably become yet more bulky than
the uname-switching you have in mind (presuming you don't want to hide
the functionality in a function or so). Something like
TZ='Europe/Prague' awk -v t="$(TZ='Europe/London' date +'%Y %m %d %H %M
%S')" 'BEGIN { print strftime("%H%M %Y-%m-%d", mktime(t)) }'
Janis
On 31.10.2017 06:49, Ian Zimmerman wrote:
> I'll be satisfied if it is portable between GNU/Linux and FreeBSD.
>
> Both systems' date(1) command provides a way to format and print an
> arbitrary time, not just the current time. But unfortunately each of
> them requires very different flags for date(1) to do that.
>
> Here's what I do now on GNU/Linux (simplified):
>
> $ date -d 'TZ="Europe/Prague" 2017-10-31 15:45' +'%H%M %m/%d/%Y'
> 0745 10/31/2017
>
> But on FreeBSD, the -d option does something completely different, and
> instead one is apparently supposed to just pass a bare time value (as if
> setting a new system time), and include the -j flag (which OTOH is
> absent in the GNU version).
>
> If there's no other way, I'll switch on the output of uname -s, but I'd
> really like to avoid that.
its time functions. But that will probably become yet more bulky than
the uname-switching you have in mind (presuming you don't want to hide
the functionality in a function or so). Something like
TZ='Europe/Prague' awk -v t="$(TZ='Europe/London' date +'%Y %m %d %H %M
%S')" 'BEGIN { print strftime("%H%M %Y-%m-%d", mktime(t)) }'
Janis
Ian Zimmerman
Oct 31, 2017, 11:34:35 AM10/31/17
to
On 2017-10-31 09:52, Bit Twister wrote:
> does FreeBSD's date support the long switch option. --date?
Of course not; they're opposed to that form of options in general.
> does FreeBSD's date support the long switch option. --date?
Thomas 'PointedEars' Lahn
Nov 1, 2017, 12:31:16 PM11/1/17
to
Ian Zimmerman wrote:
> I'll be satisfied if it is portable between GNU/Linux and FreeBSD.
>
> Both systems' date(1) command provides a way to format and print an
> arbitrary time, not just the current time. But unfortunately each of
> them requires very different flags for date(1) to do that.
>
> Here's what I do now on GNU/Linux (simplified):
>
> $ date -d 'TZ="Europe/Prague" 2017-10-31 15:45' +'%H%M %m/%d/%Y'
> 0745 10/31/2017
>
> But on FreeBSD, the -d option does something completely different, and
> instead one is apparently supposed to just pass a bare time value (as if
> setting a new system time), and include the -j flag (which OTOH is absent
> in the GNU version).
ACK, see also <https://www.freebsd.org/cgi/man.cgi?date>.
> I'll be satisfied if it is portable between GNU/Linux and FreeBSD.
>
> Both systems' date(1) command provides a way to format and print an
> arbitrary time, not just the current time. But unfortunately each of
> them requires very different flags for date(1) to do that.
>
> Here's what I do now on GNU/Linux (simplified):
>
> $ date -d 'TZ="Europe/Prague" 2017-10-31 15:45' +'%H%M %m/%d/%Y'
> 0745 10/31/2017
>
> But on FreeBSD, the -d option does something completely different, and
> instead one is apparently supposed to just pass a bare time value (as if
> setting a new system time), and include the -j flag (which OTOH is absent
> in the GNU version).
The “-d” and “-j” options/switches (not: flags) are not specified in
POSIX1-2008, which is indicative of non-portability. Unless you are
comfortable with temporarily *setting* the system time, there is
apparently no possibility that is POSIX-compliant:
<http://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html>
> If there's no other way, I'll switch on the output of uname -s, but I'd
> really like to avoid that.
instead.
--
PointedEars
Twitter: @PointedEars2
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