Omitting tzinfo argument to Trunc & Extract with USE_TZ = True and TIME_ZONE != UTC creates ambiguity for migrations

[Jacob Walls]

The Trunc() and Extract() database functions apply timezone conversions if USE_TZ = True. When the tzinfo argument is omitted, the timezone is inferred from settings.TIME_ZONE. If these functions are used in a db_default expression, this information is not captured by migrations, meaning that if settings.TIME_ZONE changes over the life of a project, then the database may never receive a corresponding update.

from django.db import models
from django.db.models.functions import ExtractHour, Now

class Person(models.Model):
    hour = models.IntegerField(db_default=ExtractHour(Now()))

To reproduce:

• Change settings.TIME_ZONE to "America/Chicago".
• Make migrations, notice no migration generated.
• Emulating the database-default python-side (as SQLite must do sometimes, see rest of linked forum post) now no longer produces the same value as the database.

In this ​forum reply, there was an idea to implement Trunc/Extract.deconstruct() to deprecate omitting tzinfo if serialized into a migration and eventually make it default to get_current_timezone() when not provided.

[Natalia Bidart]

Thank you! The forum post was very educational.