I think whoever set up the 'Scheduling' clock for WordPress did not do it correctly.
If you want something posted at 3am, then the hour should read 3. But it won't post until 3pm, which is really 15.00.
I have realized that I have to use it 'incorrectly' for the post to be posted 'correctly.'
Please note: 'In the 24-hour time notation, the day begins at midnight, 00:00, and the last minute of the day begins at 23:59. Where convenient, the notation 24:00 may also be used to refer to midnight at the end of a given date — that is, 24:00 of one day is the same time as 00:00 of the following day.'