I just arrived on this topic searching for ideas about improving the admin datetime format.
I noticed that datetime values are passed between to/from the admin form using the format defined in settings, but as you mentioned, this could be a problem when users from different timezones are using the admin interface.
I'd like to contribute to this, and I can suggest to use an iso format (used in input type "date", "datetime-local" and "time") to pass data, so all formatting problems can be avoided.
My idea was to change the `localize_input` function to return `value.isoformat()` when `isinstance(value, (datetime.datetime, datetime.date, datetime.time))`.
After this we can customize the attrs of `AdminDateWidget`, `AdminTimeWidget` (and eventually add a `AdminDateTimeWidget`)
passing the correct input type (`date`, `time` or `datetime-local`).
Any other idea?