The web's DateTime object does not support microseconds, so calculations like the one done for endOfMonth:

don't work correctly. It gets the next month and subtracts one microsecond, which ends up doing nothing on the web.

Example: If d is a DateTime representing 2024-01-05T23:52, then d.endOfMonth results in 2024-02-01T00:00:00, rather than the expected 2024-01-31T23:59:59.999.

Also, using local time by default (by constructing a new DateTIme not in utc), is probably a bad choice. If my original DateTime was in UTC, this will give incorrect results.