Scheduled messages do not send at the set time when daylight saving time changes

[Natanji]

• [x ] I have searched open and closed issues for duplicates
• [x ] I am submitting a bug report for existing functionality that does not work as intended
• [x ] I have read https://github.com/signalapp/Signal-Android/wiki/Submitting-useful-bug-reports
• [ x] This isn't a feature request or a discussion topic

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Bug description

When long-pressing on the "send message" button, a context menu opens to schedule the message for later that same day, or the next day. By default there are three options displayed that are a few hours into the future, like "8:00 tomorrow", "12:00 tomorrow" and "18:00 tomorrow". There is no mention of timezones in this quick choice menu, contrary to when the user selects the fourth option, "set date and time".

When a user selects such an option, they expect that this message will be sent when the clock strikes 8, 12 or 18 the next day.

But the message gets sent an hour later or earlier than that when a daylight saving time change occurs in between today and tomorrow.

In the #12863, @greyson-signal made the argument that users were informed about the timezone in the dialog, however, this is only the case in the detailed ("set date and time") menu, in which a timezone is displayed above. I am reporting this as a new issue because in the quick menu there is absolutely zero mention of a timezone, and so the user's expectation will in 99% of cases be that Signal will honor their local phone's time (that gets automatically changed).

I would additionally argue that a timezone change due to daylight saving time affects significantly more users than a timezone change due to travel. Virtually every Signal user has a timezone change due to DST twice a year, only a comparatively tiny fraction of Signal users travel to a different time zone and back every year.

Since Signal can easily detect whether there is a DST change between now and the scheduled time of the message, I propose that in such a case, when this time is selected in the quick menu, a new dialog pops up simply asking the user in which of the two time zones they mean to schedule the message.

Last but not least, while technical users are aware that a DST change changes their timezone, the average Joe probably isn't even aware of that; they would say "I stay in the same spot, so my timezone is the same (e.g."CET"), just sometimes with DST on or off". That this is set as two entirely different timezones on a technical level isn't obvious. So as an improvement, the detailed "set date and time" menu could also display a warning in case a DST change is detected between now and the scheduled time of the message.

Since scheduled messages can be time-sensitive notifications to other people, it would be very good to fix this issue. Doesn't mean to implement some new heuristic on what the user might mean, just make them aware and give them the choice.

Steps to reproduce

• set phone to after 21:00 in a timezone on which daylight saving time change to summer time occurs in the next night (e.g. Europe/Berlin on 30.03.2024 at 23:00)
• schedule a message via the quick menu to a time the next day (e.g. 8:00 the next day)
• wait until message gets delivered the next day

Actual result: The message is sent an hour earlier or later relative to the phone's clock that automatically changed daylight saving time (in the above example, it is only sent at 9:00)
Expected result: The message should have been sent when the phone clock struck 8:00

Device info

Signal version: 7.2.1