FW: Questions

[Fan_Zhenqiang at emc.com]

Hi Paul,
Thank you so much for replying my mail.

What if a time zone is specified together with the date/time value? For
instance, is there any function that call whether "2007-11-04 01:30 PST"
is DST?

My current project needs to do conversions between local time and UTC.
For date/time between 1970 and 2038, converting local time (in struct
tm) to UTC can be done by calling mktime() and then gmtime(). But
converting UTC to local time is not that straight forward. In addition,
for date/time falling outside of (1970, 2038), we have no way to
determine whether the given data/time value is DST or not and thus
cannot do conversion correctly. That's why I was asking the original
question. Any ideas would be sincerely appreciated.

Zhenqiang Fan


-----Original Message-----
From: Paul Eggert [mailto:eggert at CS.UCLA.EDU] 
Sent: Tuesday, April 03, 2007 4:30 PM
To: Fan, Zhenqiang
Cc: tz at elsie.nci.nih.gov
Subject: Re: FW: Questions

> From: Fan_Zhenqiang at emc.com [mailto:Fan_Zhenqiang at emc.com] 
> Sent: Tuesday, April 03, 2007 3:21 PM

> Is there a function (in any of these source files) that takes a
> date/time value and tells whether the date/time is daylight saving
time?

If T is a time_t value, and if localtime(&T) returns non-null, then
localtime(&T)->tm_isdst should tell you what you want to know (about T).

If the date/time value is some string like "2007-11-04 01:30" then
you're out of luck, because that time stamp is ambiguous in most of
North America: it might be DST, and it might be standard time.