I have a suggestion to make and it is the idea of the game time but additional to just show the date number. For example:
If someone's world is in day 1000, you can't use the java simple date format to display day 1000, only the day number according to the year or a month.
So, my suggestion is that you can put days played (day counter) similar to the Game Time option.
Many thanks for considering my request.

Thank you I didn't know that there is an extension for that

You can already do that, a J displays the in-world day number, as an extension to the SimpleDateFormat letters.