The times YmerNode.Messages reads and writes: the since a read takes, and every time an answer carries. The zone is handed in, never read here, so each rule below runs in a test against a zone with summer time.

A since is one ISO-8601 field. A span starts with P — PT1H, P1D — and reaches that far back from the moment of the call; one that nets forward — P-1D — is refused. Anything else is an instant: with an offset or Z it means exactly that; without one it is read in the zone given, which the process passes as the node time zone. An offset-less instant the clock passes twice — in the hour a fall-back repeats — is its first pass, as it is for a schedule's lifetime (YmerNode.Schedules.Lifetime); one the clock never shows — in the hour a spring-forward skips — is refused. A bare date is refused too, as neither shape.

Every time an answer carries is ISO-8601 in the zone with its offset, the way YmerNode.Schedules renders.