Should DateTime.diff/3 round numbers?

Questions & Help Questions
1

I noticed some weird anomalies in a dataset.
Turns out I was mixing Postgres and Elixir numbers.

DateTime.diff(~U[2026-03-18 05:50:08.653909Z], ~U[2026-03-18 05:50:04.655608Z], :second)
# 3

DateTime.diff(~U[2026-03-18 05:50:08.653909Z], ~U[2026-03-18 05:50:04.655608Z], :millisecond)
# 3998

select extract(epoch from ('2026-03-18 05:50:08.653909Z'::timestamptz - '2026-03-18 05:50:04.655608Z'::timestamptz))::integer
# 4

The Postgres number (4) seems reasonable.