A comparison function for date-times that is suitable as a [type@GLib.
CompareFunc].
This effectively converts both date-times to the same time zone before comparing, so date-times in different time zones can compare equal if they refer to the same instant. See [method@GLib.DateTime.difference].
Both date-times must be non-`NULL`.
| this |
first date-time to compare |
| dt2 |
second date-time to compare |
|
`-1`, `0` or `1` if this is less than, equal to or greater than |