Re: [Boost-users] [DateTime] from time_t to ptime and back - or not

23 Aug 2010

      John B. Turpish wrote:

Thank you.
...
On Fri, Aug 20, 2010 at 11:23 AM, Christoph Duelli 
wrote:
...
Code:
time_t now = time(NULL);
boost::posix_time::ptime pt = boost::posix_time::from_time_t(now);
struct tm tm = to_tm(pt);
time_t should_be_now = mktime(&tm);
BOOST_CHECK_EQUAL(now, should_be_now);
fails for me: now is by 7200 seconds (i.e. 2 hours) ahead of.
Is this a bug in date_time or am I just failing to use it properly?
I am by no means an expert, so take this with a grain of salt.
[snip]
mktime, on the other hand, assumes the tm represents your local time
(http://cplusplus.com/reference/clibrary/ctime/mktime/).
...
Whether I'm right
or wrong should be easily testable, especially for you (since you know
...
what timezone your PC is set to). Add to your program a few output lines
showing members of the tm and see if it looks right for your local time.
However, that does not really help, because the struct tm that Boost ptime
Yes, with t = time(0)
  t == mktime(localtime(t))
but
  t != mktime(gmtime(t))

produces is different from both gmtime and localtime:
DUMP gmtime
     t.tm_sec=50
     t.tm_min=31
     t.tm_hour=9
     t.tm_mday=23
     t.tm_mon=7
     t.tm_year=110
     t.tm_wday=1
     t.tm_yday=234
     t.tm_isdst=0
     t.tm_gmtoff=0
     t.tm_zone=GMT
diff(time,mktime(gmtime(time)))=3600
DUMP localtime
     t.tm_sec=50
     t.tm_min=31
     t.tm_hour=11
     t.tm_mday=23
     t.tm_mon=7
     t.tm_year=110
     t.tm_wday=1
     t.tm_yday=234
     t.tm_isdst=1
     t.tm_gmtoff=7200
     t.tm_zone=CEST
diff(time,mktime(localtime(time)))=0
DUMP boost from_time_t -> to_tm
     t.tm_sec=50
     t.tm_min=31
     t.tm_hour=9
     t.tm_mday=23
     t.tm_mon=7
     t.tm_year=110
     t.tm_wday=1
     t.tm_yday=234
     t.tm_isdst=-1
     t.tm_gmtoff=0
     t.tm_zone=NULL
diff(time,mktime(to_tm(from_time_t(time))))=7200

It is not obvious how to fix the tm obtained from to_tm.
Probably it is easier to fix the resulting time_t.

I guess I will settle for the approach given in
  http://lists.boost.org/boost-users/2007/07/29567.php
and stop using from_time_t and to_tm

Best regards
Christoph