Support for timestamp trigger notifications across timezones

[samueljamesbell]

Hello! Just want to start by saying how much I love notifee - you guys are doing a great job!

In our production app, we're seeing reports of notifications not arriving at correct times when the user has changed timezone. We're making extensive use of TimestampTrigger in order to set notifications at a particular point in the future in terms of milliseconds since the (UTC) epoch.

Here's an example of what we're seeing:

1. Notification created at 13:00 GMT (+00:00), with trigger set for 20:00 GMT, i.e. 7 hours in the future
2. User changes timezone, e.g. to CET (+01:00)
3. Notification arrives at 20:00 CET (i.e. 19:00 GMT), rather than 19:00 CET (i.e. 20:00 GMT), i.e. only 6 hours after the creation time

Given that TimestampTrigger accepts a timestamp argument in terms of milliseconds since the epoch, this isn't what I was expecting.

I have had a dig through the notifee source, and it seems that, on iOS at least, TimestampTrigger ultimately relies on matching DateComponents, rather than a milliseconds since epoch trigger (see here). It also looks like UNCalendarNotificationTrigger could potentially accept a DateComponents with a timezone attached.

Here's how we're scheduling, for reference:

let notification: Notification = {
  title,
  body,
  ...
};

let trigger: TimestampTrigger = {
  type: TriggerType.TIMESTAMP,
  timestamp: timestampInMillisSinceEpochUTC,
  alarmManager: {
    allowWhileIdle: true,
  },
};

return await notifee.createTriggerNotification(notification, trigger);

Would be great to understand if I'm missing something here. Does notifee have support for notifications that are fixed to a UTC time, rather than vary with the user's current timezone?

Thanks in advance for your help!