KevinTheJedi Found the cause of the issue 😎
The date/time fields were in a custom form, and upon an agent trying to edit the entered value, it was a year in advance. However, going back to edit the form...
If I have the 'Timezone Aware' option selected, everything works as expected! 😃 It's only an issue if this box is not selected. The server and all of our agents are in the same timezone, so i wouldn't have thought this would make any difference 🤔 Certainly not a year anyway!
Thanks for looking into it anyway - hopefully this helps resolve the issue.
Tagging @dturcott who may appreciate this resolution too.
EDIT: osTicket seemed to add those default values automagically after I hit save. Given it's (frustratingly) possible start dates will be in the past, I set this back to "0". Still works as described above.