[Development] QDateTime::currentDateTime().toString(Qt::SystemLocaleLongDate) returns as if the DST was still in effect

Mitch Curtis mitch.curtis at digia.com
Mon Oct 29 16:51:03 CET 2012

On Monday, October 29, 2012 04:28:17 PM Jan Kundrát wrote:
> However, `mitchc` on #qt kindly confirms that he also gets "CEST" in
> QDateTime's output, even after a reboot since the DST change. He says that
> brokenDown's tm_isdst is *not* set for him.
Mine looks like: 

tm_gmtoff = 3600
tm_hour	 = 16
tm_isdst	 =  	0
tm_mday = 29
tm_min = 36
tm_mon	 = 9
tm_sec = 9
tm_wday = 1
tm_yday	 = 302
tm_year	 = 112
tm_zone	 = "CET"

> So, given that `date` reports the time correctly, I suspect that Qt is using
> some strange way of obtaining the TZ information. Do you have any ideas
> about what is confusing it to believe that DST is still active on this box?
> Please note that the actual hours being displayed are always correct, it's
> just the TZ indication which is off.

The problem seems to be that localtime_r fetches the correct timezone, but the 
timeZone() function in qlocale.cpp (which is used when you use a date format 
like Qt::SystemLocaleLongDate) returns a hard-coded tzname[1], which is always 
going to be a timezone with DST in effect:

"The array tzname contains two strings, which are the standard names of the 
pair of time zones (standard and Daylight Saving) that the user has selected. 
tzname[0] is the name of the standard time zone (for example, "EST"), and 
tzname[1] is the name for the time zone when Daylight Saving Time is in use 
(for example, "EDT"). These correspond to the std and dst strings 
(respectively) from the TZ environment variable. If Daylight Saving Time is 
never used, tzname[1] is the empty string. " [1]

I vaguely remember running into a similar issue with https://codereview.qt-
project.org/#change,35242, but I think it's a known issue that timezone 
support in Qt is not up to scratch. John Layt is working on the ICU backend 
for this stuff though [2], so it will be improved in the not-too-distant 

[1] http://www.gnu.org/software/libc/manual/html_node/Time-Zone-Functions.html
[2] http://www.layt.net/john/blog/odysseus/icu_did_u_c_me
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