from datetime import timedelta

from pytz import timezone, utc, UnknownTimeZoneError

from django.conf import settings

local_tz = timezone(settings.TIME_ZONE)

def request_tz_convert(date, request):

    return tz_convert(date, request.session.get('tz',None))

def tz_convert(date, tz):

    """

    Convert date to time zone

    tz can be;

        - '-2' (relative to utc)

        - 'Paris/Europe' real timezone like (cf pytz)

    """

    if tz:

        system_local_date = local_tz.localize(date)

        try:

             # simple utc delta?

            utc_offset = int(tz)

            utc_time = system_local_date.astimezone(utc)

            res = utc.normalize(utc_time + timedelta(hours=utc_offset))

            return res

        except:

            try:

                # real timezone

                timez = timezone(tz)

                local_date = system_local_date.astimezone(timez)

                return local_date

            except UnknownTimeZoneError:

                # fall back to date

                return date

    else:

        return date