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Re: [Help-glpk] Time conversion functions

From: glpk xypron
Subject: Re: [Help-glpk] Time conversion functions
Date: Wed, 26 Nov 2008 05:36:56 +0100

Hello Andrew,

>> It is *not* defined whether t is UTC, local time, or somewhat else.
If only function str2time() and time2str() exist this is valid.

>> but, in my opinion, using the daylight save time in modeling is an extremely 
>> bad idea
You proposed to add a function time() to obtain the current time, which I 
appreciate. When implementing it you have to decide in which time zone the 
return value is.

My suggestion is that this function time() should return a value in UTC. This 
can be achieved using C99 functions time() and gmtime(). (In your current 
implementation you use localtime()).

If function time() were in local time it would have discontinuities due to day 
light saving time, which as you stated should be avoided.

>> If in your model t is UTC, and
>> you need to convert it to a local time, you should add the time offset
>> which takes into account the timezone and daylight saving info and is
>> defined in your model.

If my data is relative to UTC the ISO 8601 time stamps may look like this
2008-11-26T05:00:00Z - Z indicating that the time zone is UTC or like this
2008-11-26T06:00:00+0001 - refering to the same time, +0001 indicating that the 
time zone is UTC + 1 hour. (If data is in [unspecified] local time there will 
be no Z and no offset.)

If I have data in the ISO format with different offsets relative to UTC I would 
like to be able parse the offset. 

Parsing the offset in GMPL would be quite tedious.

Hence I suggest to add a format code '%z' to function str2time(). %z is used 
for the same purpose in the C99 function strftime().

For output it is sufficient to add the offset manually:
printf "The following timestamps are equal:\n";
printf time2str(t, "%FT%TZ\n");
printf time2str(t+3600, "%FT%T+0100\n");

The following formats of offsets are allowed by ISO 8601:


Best regards


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