UpdateReminder

Change one of the current user's reminders: reword it, move when it fires, or mark it done. Get the `reminderId` from ListReminders or CreateReminder. Pass only the fields you are changing; anything omitted is left alone. At least one must be given. - Snooze or reschedule: set `remindAt` - Mark done: `completed: true` — the user has dealt with it and it leaves their active list - Reopen: `completed: false` - Reword: set `description` Marking done is the normal way to clear a reminder the user has handled, and it is reversible: the reminder drops out of the default ListReminders results but is still there, readable with `completed: true` and restorable with `completed: false`. Reach for DeleteReminder only when the user wants the reminder not to exist; that cannot be undone. Two things this tool will not do. It cannot change what a reminder is attached to — create a new reminder and delete this one instead. And setting `remindAt` on a repeating reminder replaces the repetition with that single firing, so only do it if the user asked to stop it repeating. ## Times are UTC — convert both ways Timestamps are absolute instants, in and out, while the user asks in their own timezone. Getting this wrong silently sets the reminder to the wrong hour. - **In:** resolve their wording against their local time, then convert. For America/New_York (UTC-4 in August), \"3pm tomorrow\" on 2026-08-12 is `\"2026-08-13T19:00:00Z\"`, not `\"2026-08-13T15:00:00Z\"`. - **Out:** report the response's UTC value back in their timezone — `\"2026-08-13T19:00:00Z\"` is \"3:00 PM tomorrow\". Ask for their timezone rather than assuming UTC.