python - How do I get the visitor's current timezone then convert timezone.now() to string of the local time in Django 1.4?

Question

I understand that the best practice now with Django 1.4 is to store all datetime in UTC and I agree with that. I also understand that all timezone conversation should be done in the template level like this:

{% load tz %}

{% timezone "Europe/Paris" %}
    Paris time: {{ value }}
{% endtimezone %}

However, I need to convert the UTC time to the request's local time all in Python. I can't use the template tags since I am returning the string in JSON using Ajax (more specifically Dajaxice).

Currently this is my code ajax.py:

# checked is from the checkbox's this.value (Javascript).
datetime = timezone.now() if checked else None

$ order_pk is sent to the Ajax function.
order = Order.objects.get(pk=order_pk)
order.time = datetime
order.save()

return simplejson.dumps({
    'error': False,
    'datetime': dateformat.format(datetime, 'F j, Y, P') if checked else 'None'
})

So even if the current time is April 14, 2012, 5:52 p.m. in EST time (my local timezone), the JSON response will return April 14, 2012, 9:52 p.m, because that is the UTC time.

Also I noticed that Django stores a template variable called TIME_ZONE for each request (not actually part of the request variable), so since my is America/New_York, I'm assuming that Django can figure out each visitor's own local timezone (based on HTTP header)?

Anyway, so my question is two-fold:

1. How do I get the visitor's local timezone in my ajax.py? (Probably pass it as a string argument like {{ TIME_ZONE }})
2. With the visitor's local timezone, how to convert the UTC timezone.now() to the local timezone and output as a string using Django's dateformat?

EDIT: for @agf

timezone.now() gives the UTC time when USE_TZ = True:

# From django.utils.timezone
def now():
    """
    Returns an aware or naive datetime.datetime, depending on settings.USE_TZ.
    """
    if settings.USE_TZ:
        # timeit shows that datetime.now(tz=utc) is 24% slower
        return datetime.utcnow().replace(tzinfo=utc)
    else:
        return datetime.now()

Is there anyway to convert a datetime to something other than UTC? For example, can I do something like current_time = timezone.now(), then current_time.replace(tzinfo=est) (EST = Eastern Standard Time)?

Answer 1

You need to read the Django Timezones docs carefully.

One important point:

there's no equivalent of the Accept-Language HTTP header that Django could use to determine the user's time zone automatically.

You have to ask the user what their timezone is or just use a default.

You also need to make sure:

USE_TZ = True

in your settings.py.

Once you have a timezone tz, you can:

from django.utils import timezone

timezone.activate(tz)

datetime = timezone.now() if checked else None

to get a timezone-aware datetime object in timezone tz.