Timezones: store UTC, decide in theirs

Store instants in UTC. Store the person's IANA timezone as a separate field. Convert at the two edges only: what a human reads, and whether it is a decent hour to send them something.

The recurring schedule bug

"Every weekday at 09:00" is not an instant. Stored as one, it is correct until the clocks change and then it is 08:00 or 10:00 forever. Store the rule - local time plus a zone - and compute the next instant each time.

# Wrong: an instant, computed once.
next_run = datetime(2026, 9, 12, 6, 0, tzinfo=timezone.utc)

# Right: the rule, resolved each time in the zone it belongs to.
zone = ZoneInfo(account.timezone)          # "Africa/Kampala"
local = datetime.now(zone).replace(hour=9, minute=0, second=0)
next_run = (local + timedelta(days=1)).astimezone(timezone.utc)

Quiet hours are the one that matters

Order events fire whenever the warehouse system runs. Without a quiet-hours rule in the recipient's zone, somebody's phone lights up at 03:00 with a notification about a parcel. It is the fastest way to convert a useful channel into a blocked number.

1. Define quiet hours per account, in local time.
2. Hold anything non-urgent until the window opens.
3. Decide explicitly what is urgent enough to override - a security code usually is, a delivery update is not.
4. Never let a backlog release all at once when the window opens. Four hundred messages at 08:00 is its own incident.

Where the zone comes from

• Ask the account for their business timezone. This is the one to trust for quiet hours.
• Infer from the phone number's country as a fallback for recipients, knowing it is often wrong for people who travel.
• Never guess from the server. The server's zone is a deployment detail and has no relationship to anybody.

Display, and the small kindness

Show times in the reader's zone, and say which zone when it could be ambiguous. "Delivered 14:32" is fine in a conversation; "Scheduled for 09:00 EAT" is what a person setting a campaign needs, because they are about to message people in another country.