lightbox: Use a friendlier format for the date, like "Yesterday at 4:47 PM"

[chrisbobbe]

…or "A few seconds ago", etc.

Currently it shows something like "Mar 31, 2023 15:09:51".

Implementation (full solution)

It's possible that we'll want to merge a partial solution before implementing this, but anyway I wanted to write down some findings.

Dart intl.DateFormat doesn't support some desired features:

1. Relative times, like "21 minutes ago"
2. Combining a date and time into one string with a localized separator, such as " at " in English, " um " in German, or " " in Japanese.

We could work around, with our own code or maybe with a library in Dart (such as timeago or relative_time). But (1) is slightly tricky to implement, and requires giving more plural-syntax strings to our volunteer translators. For (2) we could just give the separator to our translators, but that feels unsatisfying because it shouldn't be necessary.

I propose using an ICU library via ffi, which we already plan to do for other localization features: #1165 (comment)

• If the timestamp was today, show the time only, using DateFormat::createTimeInstance with style RELATIVE_SHORT or RELATIVE_MEDIUM. It should produce e.g. "21 minutes ago".

Otherwise, use DateFormat::createInstanceForSkeleton, passing an appropriate "skeleton" (see this helpful explainer):

• If the timestamp was yesterday, say "Yesterday at {time}". For time, use skeleton jmm, for "12:15 PM".
• If the timestamp was before yesterday and in the current year, omit the year; use skeleton MMMdjmm, for "Jan 2 at 5:00 PM"
• If the timestamp was before the current year, include the year, using skeleton yMMMdjmm, for "Jan 2, 2024 at 5:00 PM".
• If the timestamp is in the future (maybe the server's clock is wrong?), also use skeleton yMMMdjmm. A relative time like "in 21 minutes" would be awkward here.

The strings must be formatted with the same time zone and calendar (Gregorian, Chinese lunar, etc.) as we use to decide which day or year the timestamp is on. We should be able to choose an appropriate Calendar for the which-day/year calculation and pass that Calendar to DateFormat::createInstanceForSkeleton.

We should periodically force-rebuild the widget that shows the time, to avoid freezing with text like "1 minute ago".

We should offer a tooltip that shows the full date and time, with seconds, plus the timezone; skeleton yMMMdjmmsszzzz.

[mofirojean]

@gnprice can i work on this?

[gnprice]

Sure. Do see our guide to picking an issue:
https://zulip.readthedocs.io/en/latest/contributing/contributing.html#picking-an-issue-to-work-on

so in particular before claiming an issue, you should have already gotten started working on it, finding the relevant piece of code and getting an idea of how you'll approach it.

Also note this issue is in the "Post-launch" milestone, not "Launch" or earlier. That means the core team will have limited time to spend helping with this issue for the next few months. But I think the issue is simple enough that that should be fine.