Function stamp 

pub fn stamp(at: DateTime<Utc>) -> String

Render a timestamp the one way the store compares them.

Fire times are compared lexicographically by the claim statement’s WHERE last_fired_at < ?, which is only equivalent to chronological order if every timestamp has the same fixed width and zone. Whole seconds and a literal Z guarantee that; a stray +00:00 or fractional-second stamp would silently break the ordering that keeps a tick from firing twice.