Python time.timezone / time.altzone edge case

While browsing the log of one of my Bazaar branches, I noticed that the commit messages were being recorded as occurring in the +0800 time zone even though WA switched over to daylight savings.

Bazaar stores commit dates as a standard UNIX seconds since epoch value and a time zone offset in seconds. So the problem was with the way that time zone offset was recorded. The code in bzrlib that calculates the offset looks like this:

def local_time_offset(t=None):
    """Return offset of local zone from GMT, either at present or at time t."""
    # python2.3 localtime() can't take None
    if t is None:
        t = time.time()

    if time.localtime(t).tm_isdst and time.daylight:
        return -time.altzone
        return -time.timezone

Now the tm_isdst flag was definitely being set on the time value, so it must have something to do with one of the time module constants being used in the function. Looking at the values, I was surprised:

>>> time.timezone
>>> time.altzone
>>> time.daylight

So the time module thinks that I don’t have daylight saving, and the alternative time zone has the same offset as the main time zone (+0800). This seems a bit weird since time.localtime() says that the time value is in daylight saving time.

Looking at the Python source code, the way these variables are calculated on Linux systems goes something like this:

  1. Get the current time as seconds since the epoch.
  2. Round this to the nearest year (365 days plus 6 hours, to be exact).
  3. Pass this value to localtime(), and record the tm_gmtoff value from the resulting struct tm.
  4. Add half a year to the rounded seconds since epoch, and pass that to localtime(), recording the tm_gmtoff value.
  5. The earlier of the two offsets is stored as time.timezone and the later as time.altzone. If these two offsets differ, then time.daylight is set to True.

Unfortunately, the UTC offset used in Perth at the beginning of 2006 and the middle of 2006 was +0800, so +0800 gets recorded as the daylight saving time zone too. In the new year, the problem should correct itself, but this highlights the problem of relying on these constants.

Unfortunately, the time.localtime() function from the Python standard library does not expose tm_gmtoff, so there isn’t an easy way to correctly calculate this value.

With the patch I did for pytz to parse binary time zone files, it would be possible to use the /etc/localtime zone file with the Python datetime module without much trouble, so that’s one option. It would be nice if the Python standard library provided an easy way to get this information though.