An Ecto.Type storing a Localize.Duration.t/0 in a PostgreSQL interval column.

A Localize.Duration is an amount of time in calendar units — a year, three months, two days — and interval is the PostgreSQL type for exactly that. Storing one as an interval rather than a number of seconds keeps the calendar meaning that "one month" carries (a month is not always thirty days), and lets the database add an interval to a date or sum a column of them.

Schema

# in a migration
add :billing_period, :interval

# in a schema
field :billing_period, Localize.Duration.Ecto.Type

Normalization

PostgreSQL interval keeps three components — months, days and time — so the units within each component are interchangeable: a duration of ninety minutes is stored as, and loaded back as, one hour and thirty minutes, and thirteen months as one year and one month. Durations that are already in their normal form round-trip unchanged. The day and month components are kept distinct — a month is never folded into days — because PostgreSQL applies them separately when an interval is added to a date.