Localise notification email deadlines to each recipient's timezone

[alexeygrigorev]

Background

Notification emails that show a deadline (currently the new peer-review-assignment email, and a good fit for deadline reminders too) render the deadline in UTC — e.g. "Thursday, 2 July 2026, 18:00 UTC".

We initially tried showing a fixed set of zones (PT/ET/London/Berlin/IST) but dropped it as noisy. For now we stick to UTC.

Goal

Show the deadline in each recipient's own timezone, similar to what the AI shipping lab does, so a learner sees the deadline in their local time without mental math.

Notes / considerations

• The deadline is formatted in course_management/deadlines.py → format_deadline_for_email() (CMP side). Today it returns a single UTC string.
• To localise per recipient we need each user's timezone. Options:
  • Store a timezone on the user profile (let users set it, or infer from browser/IP at signup).
  • Since these emails go out as a Datamailer recipient-list send (one template, per-member context merged), per-recipient rendering means either (a) CMP includes each member's tz + a pre-formatted local string in member metadata, or (b) Datamailer renders the tz at send time. Prefer (a) to keep Datamailer platform-agnostic.
• Fall back to UTC when a recipient has no timezone set.
• Keep the weekday in the output.

Out of scope

Per-recipient localisation of all emails — start with the deadline-bearing ones.

[alexeygrigorev]

Implemented on main. Deadline-bearing notification emails are now localized per recipient:

• A preferred_timezone (IANA) field exists on the user, settable in account settings (accounts/forms.py — a timezone ChoiceField built from build_timezone_options()).
• accounts/services/timezones.format_deadline_for_user(deadline, user) renders the deadline in the user's saved timezone, keeping the weekday, and falls back to UTC when no timezone is set.
• Per-recipient rendering happens the preferred way (option (a) from the issue — CMP puts the localized string in each member's metadata, Datamailer stays platform-agnostic):
  • Peer-review assignment: peer_review_members.py:peer_review_assignment_deadline_metadata → format_deadline_for_user(project.peer_review_due_date, submission.student).
  • Deadline reminders: courses/deadline_reminder_members.py:deadline_metadata → format_deadline_for_user(deadline, user).
• Templates consume the per-member field ({{ deadline_summary }} for peer review, {{ deadline_at }} for reminders).

Tested: courses/tests/deadline_reminder_homework.py asserts a Europe/Berlin recipient gets Thursday, 18 June 2026, 01:00 Europe/Berlin (+ deadline_timezone), test_deadlines.py asserts the UTC fallback, and test_datamailer_templates.py checks the localized render. Closing as done.