It’s about time

by Aymeric Augustin

Note

Had to deal with a business thing so didn’t get all of Aymeric’s talk down. What I got was from some really awesome material.

RFC 3339

  • Current era
  • Stated offset
  • universal time
  • instant in time
from datetime import datetime
datetime(
    year=2012, month=6, day=5
    hour=16, minute=10, second-0,
    microsecond=0,
    tzinfo=FixedOffset(120)
)

Time zones add complexity

aware vs naive datetimes

>>> naive = datetime(2012, 6, 5, 16, 15)
>>> tz = timzeone("Europe/Paris")
>>> aware - tz.localize(naive)
>>> naive - aware

From the Python docs: Whether a naive datetime object represents UTC, local time, or time in some other timezone is purely up to the program.

DST transitions

from datetime import datetime

dates and datetimes

  • dates are always naive
  • they don’t suffer from the same problems as naive datetimes
  • using an aware datetime as a date is an accident waiting to happen
  • Django supports mixing naive datetimes and dates

Django >= 1.4

  • Uses aware datetimes in URC internally
  • stores naive datetime sin UTC in the database (except PostgreSQL)
  • converts to aware datetimes in local time in forms and templates
  • supports multiple time zones!

default and current time zones

  • default = settings.TIME_ZONE

    • used in models for conversions between naive and aware objects

  • current = end user’s time zone

    • used in templates and forms
    • for multiple time zones support

auto-conversions

  • ensure backwards compatibility

  • avoid surprises for single time zone sites

  • but support sloppy contructs e.g.,

    • filter a DateTimeField with a date
    • save a datetime in a DateField

Utilities

>>> from django.conf import settings
>>> from django.utils import timezone

>>> settings.USE_TZ = True
>>> timezone.now()
<snip>

limitations in Django 1.4

  • The database works in UTC (ticket #17260)

  • QuerySet.dates()

    • __year/month/day/week_day

  • Author of pluggable apps may have to handle two cases

Key learnings

  1. A datetime is a point in time. A date is a calendaring concept.
  2. Use aware datetimes in UTC and convert to local time for humans.
  3. learn how to use pytz properly especially localize and normalize

Time isn’t as simple as it seems. Learn and practice!