Is it safe to transform dates back and forth using UTC in order to ignore DST but still using a valid locale for the current user?

Question

I have a date (no time, assuming time 00:00:00) that I convert between a time_t and a struct tm.

I get a date in the simple YYYYMMDD format and convert that to a struct tm:

struct tm my_tm;
memset(&my_tm, 0, sizeof(my_tm));
my_tm.tm_year = str.mid(0, 4).toInt() - 1900;
my_tm.tm_mon = str.mid(4, 2).toInt() - 1;
my_tm.tm_mday = str.mid(6, 2).toInt();

P.S.: for those wondering I have a QString (Qt) hence the mid() and toInt() members used.

Then I convert that date to a time_t using mktime():

time_t my_time(mktime(&my_tm));

At that point the date changes to the day before (more precisely, -1h) if the date is March 6, 2016 ("20160306" becomes 2016/03/05 in the struct tm). This is because of DST (the tm_isdst is set accordingly).

I have another version of mktime() called mkgmtime() and that works as expected: I get the same date since DST is ignored by that function:

time_t my_time(mkgmtime(&my_tm));

I can then convert the date back to a struct tm using the gmtime_r() function. This way, again, I get the same date:

struct tm other_tm;
gmtime_r(&my_time, &other_tm);

However, at some point I want to display the date in a standard format as defined by the end user's locale. For that I use the ICU library. In the following, I force f_current_timezone variable to UTC before calling format_date(), then the date comes out as Mar 6, 2016 (if I keep the user timezone, I may instead get Mar 5, 2016.)

QString locale::format_date(time_t d)
{
    QUnicodeString const timezone_id(f_current_timezone);
    LocalPointer<TimeZone> tz(TimeZone::createTimeZone(timezone_id));
    Locale const l(f_current_locale.toUtf8().data());
    LocalPointer<DateFormat> dt(DateFormat::createDateInstance(DateFormat::kDefault, l));
    dt->setTimeZone(*tz);
    UDate const udate(d * 1000LL);
    QUnicodeString u;
    dt->format(udate, u);
    return u;
}

Is it safe (Does it make sense to you) to use "UTC" as the timezone to get the correct result from format_date()?

score 0 · Answer 1 · answered Apr 03 '16 at 01:53

0

Before using mktime(), set tm_hour to 12 (12pm). If all you care about is formatting the date portion using the application's locate, using noon for mktime() will be sufficient, for that purpose.

answered Apr 03 '16 at 01:53

Sam Varshavchik

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I thought of that, but what if your timezone is +12 or -12 hours? Wouldn't that be affected? – Alexis Wilke Apr 03 '16 at 04:34
mktime uses the local timezone. It will attempt to use whatever noon in the local timezone is. – Sam Varshavchik Apr 03 '16 at 12:36

Is it safe to transform dates back and forth using UTC in order to ignore DST but still using a valid locale for the current user?

1 Answers1