Why use `utc_datetime` over `naive_datetime` for Ecto?

[dimitarvp]

@itssasanka

I read that using utc_datetime “ensures” that dates are always converted to utc before getting stored/retrieved from and to the database, however I am not sure what the significance of the “ensures” aspect here is, since naive_datetime seems to do it anyway.

This only works for automatically generated fields like inserted_at and updated_at and if you manually set other datetime fields to DateTime.utc_now(). But if you have any other Elixir code that is not timezone-aware and it fetches time in the local timezone (or have a legacy DB with non-UTC datetimes in it) then you’re in for trouble.

Better to do all of these together:

• Use utc_datetime type for datetime fields in your Ecto schema modules.
• When setting times manually, always start with DateTime.utc_now() and/or do calculations with Elixir’s stdlib or timex, always making sure you use UTC.
• If you have to work with an inherited legacy database, just make a one-off script to convert any non-UTC datetimes to UTC. It will save you a world of trouble in the future.