Good question. You would expect the game to continue where you 'left' FM23. Otherwise it would have to simulate the period in between and what does it mean for your employment in the meantime.
If it works like that, that you can start with the FM23 startdate in FM24, I wonder what it would mean if you try a next savegame into FM25. Would it mean you won't have women's football for instance since that is in the new database and not in FM23 or FM24. Would be interesting to see if you lose part of the game features that way.