Personally, I would also like to see our official datetime handling improved in this and other ways, but frankly, it’s probably not going to be a priority any time soon (I wanted some other improvements, myself, and ended up writing Zoned DateTime Picker out of impatience)
If we wait for an official implementation, it might realistically never happen. If you want this feature available before we die of old age, I could write a plugin for you? It would be similar to Computed Fields, but with relative dates, and it would fill in a specified empty date field on record open.
Datetime math is not trivial… it has to deal with:
Time zone complications (would “3 days from now” be relative to the editor’s browser time zone, UTC, our server time, the API time, the editor’s preferred zone…?) I don’t even know how it works for that current implementation…
Relative date math is itself complicated: Is “3 days from now” 72 hours, or is it 3 calendar days (from when… the current time? today’s midnight?) — and then what happens on a daylight savings time change, or a leap year, etc. The longer the period, the more edge cases that could go wrong. (Luxon: Math has a better writeup).
The Temporal API is ever just around the corner — in theory — and if/when it ever actually comes out, we’ll probably have to rewrite the whole thing.
Anyhow, if this is something you want to see soon, and having it as a plugin is acceptable, let me know the use case here and I can whip something up for you. Otherwise, we can of course leave the feature request open and hope it gets more uptake… up to you!