Django 1.6 filter by hour and time zone issue - django

Question

In my application I use time zones (USE_TZ=True) and all the dates I create in my code are aware UTC datetime objects (I use django.util.timezone.now for the current date and the following helper function to ensure all the dates in my instances are what I expect:)
#classmethod
def asUTCDate(cls, date):
if not timezone.is_aware(date):
return timezone.make_aware(date, timezone.utc)
return date.replace(tzinfo=timezone.utc)
I also enforced the check of naive/aware dates using this snippet (like suggested in the doc):
import warnings
warnings.filterwarnings(
'error', r"DateTimeField .* received a naive datetime",
RuntimeWarning, r'django\.db\.models\.fields')
As I understood so far, this is the right way to proceed (this is a quote from the django documentation: "The solution to this problem is to use UTC in the code and use local time only when interacting with end users."), and it seems that my app is handling dates very well… but I have just implemented a filter against a model that makes use of the Django 1.6 __hour and it force the extraction based on the user timezone, the result is something like:
django_datetime_extract('hour', "object"."date", Europe/Rome) = 15
but this breaks my query, since some results I was expecting are not included in the set, but when I use a __range to search between dates it seems to work as expected (objects with a date in the range are returned)… so it seems to me that Django takes into account timezones in queries only for the __hour filter… but I don't understand why… I was supposing that UTC is used everywhere except in templates where the displayed dates are formatted according to user tz, but maybe that's not true.
So my questions are: is the way I'm working with time zones right? Is __hour filter wrong or what?

Answer 1

It seems as though you're doing the right thing with dates. However, all the documentation for any date related functionality, such as filtering by hour include this note:
When USE_TZ is True, datetime fields are converted to the current time
zone before filtering.
For the range filter, this note doesn't exist because range can be used to not only filter on dates, but other types as well such as integers and characters. ie It is not necessarily datetime aware.
In essence the problem comes down to this: where do you draw the line between 'interacting with users' where times are in a local timezone, and what is internal where times are in UTC? In your case, you could imagine a user entering in a search box to search for hour==3. Does that mean for example that your form code should do the conversion between hour==3 and the UTC equivalent? This would then require a special forms.HourField. Or perhaps the value (3) should be fed directly to the query where we know that we're searching on an hour field and so a conversion is required.
We really have to follow the documentation on this one.
Values which are going to be filtered against date/time fields using any of the specialised date/time filtering functions will be treated as being in the user's local time zone.
If using the range filter for dates no time conversions occur so you are expected to convert the user's entered local time value to UTC.