HOW IT WORKS
Create once, update safely
Use a stable external event ID to upsert a planned notification. When an appointment changes, update its scheduled time rather than creating a duplicate pending job.
Cancel before dispatch
If an underlying event is cancelled, its notification must be cancelled before delivery. Once a push is accepted by a provider, recall is generally not guaranteed.
Never deliver stale reminders
Specify an expiration time. Workers should discard expired jobs and record a clear reason, rather than sending a reminder after the event has passed.
Time zones and daylight saving
Store instants in UTC, retain the originating time zone when relevant, and document how recurring schedules behave during DST changes.