Shallow Thoughts : tags : timezones

A More Time Zone Tolerant datetime Class

Yesterday I signed in to the billtracker, and got an error page when trying to display my bill list:

[ ... ]
   File "/var/www/nmbilltracker/billtracker/app/models.py", line 766, in location_html
     if self.last_action_date > self.scheduled_date.replace(tzinfo=None):
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 TypeError: can't compare offset-naive and offset-aware datetimes

Python's datetime class drives me crazy.

Any given datetime object might or might not have a timezone. Those that do are called "timezone aware" or just "aware" datetimes; those without a timezone are called "unaware" or "naive". Any given function might or might not return a timezone-aware datetime. If you ever mess up and call a function that returns a timezone when you didn't expect one, or vice versa, or if a function you call changes in that respect, now you have a hidden time bomb that will crash your program the next time you do any sort of comparison with or subtraction from another datetime, and by then, you may have no idea way of finding out where the problematic time came from so you can guard against it happening again.

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Tags: python, programming, timezones
[ 18:53 Feb 21, 2026    More programming | permalink to this entry | ]

Garmin GPX timestamp bizarreness

My last entry mentioned some work I'd done to one of my mapping programs, Ellie, to gather statistics from the track logs I get from my Garmin GPS.

In the course of working on Ellie, I discovered something phenomenally silly about the GPX files from my Garmin Vista CX, as uploaded with gpsbabel.

Track log points, quite reasonably, have time stamps in "Zulu time" (essentially the same as GMT, give or take some fraction of a second). They look like this:

<trkpt lat="35.289519913" lon="-115.227057561">
  <ele>1441.634277</ele>
  <time>2010-10-14T17:51:35Z</time>
</trkpt>

But the waypoints you set for specific points of interest, even if they're in the same GPX file, have timestamps that have no time zone at all. They look like this:

<wpt lat="35.334813371" lon="-115.178730609">
  <ele>1489.917480</ele>
  <name>001</name>
  <cmt>14-OCT-10 11:18:51AM</cmt>
  <desc>14-OCT-10 11:18:51AM</desc>
  <sym>Flag, Blue</sym>
</wpt>

Notice the waypoint's time isn't actually in a time field -- it's duplicated in two fields, cmt (comment) and desc (description). So it's not really intended to be a time stamp -- but it sure would be handy if you could use it as one.

You might be able to correlate waypoints with track points by comparing coordinates ... unless you spent more than an hour hanging around a particular location, or came back several hours later (perhaps starting and ending your hike at the same place). In that case ... you'd better know what the local time zone was, including daylight savings time.

What a silly omission, considering that the GPS obviously already knows the Zulu time and could just as easily use that!

Tags: mapping, GIS, programming, timezones
[ 22:09 Nov 03, 2010    More mapping | permalink to this entry | ]