pytz and astimezone() cannot be applied to a naive datetime

For pytz timezones, use their .localize() method to turn a naive datetime object into one with a timezone:

start_date = local_tz.localize(start_date)

For timezones without a DST transition, the .replace() method to attach a timezone to a naive datetime object should normally also work:

start_date = start_date.replace(tzinfo=local_tz)

See the localized times and date arithmetic of the pytz documentation for more details.

You could use local_tz.localize(naive_dt, is_dst=None) to convert a naive datetime object to timezone-aware one.

from datetime import datetime
import pytz

local_tz = pytz.timezone('Asia/Tokyo')

start_date = local_tz.localize(datetime(2012, 9, 27), is_dst=None)
now_utc = datetime.utcnow().replace(tzinfo=pytz.utc)

print start_date > now_utc

is_dst=None forces .localize() to raise an exception if given local time is ambiguous.

If you are using Django Rest Framework you could override the DateTimeField class like:

class DateTimeFieldOverridden(serializers.DateTimeField):

def to_representation(self, value):
    local_tz = pytz.timezone(TIME_ZONE)
    value = local_tz.localize(value)
    return super(DateTimeFieldOverridden, self).to_representation(value)

And you use it like this in your serializer:

date_time = DateTimeFieldOverridden(format='%d-%b-%Y', read_only=True)

Hope this helps someone.