The coins are still there. I guess I can try some dates
I would start with
2009-01-09 (Bitcoin's first transaction date)
2016-05-05 (the date that wallet got funded at)
Make sure you do tests offline, or if you are knowledgeable enough, on a live OS, still offline. Hacks are really a thing and 10
BTC is an overly good looking amount.
Make sure you know how your father was writing the dates (mm/dd/yyyy. dd.mm.yyyy, yyyy-mm-dd or other ways)
Beware of people that may offer you help in private, some may be scammers.
Good luck.
He could just brute-force it with a simple few lines of code. I mean there are only so many days since Bitcoin has come into existence. If you try out them all eventually you will get it. I do not think that will even take him all night. Unless he does not know how to automate the process in which case he can still spend a couple of days trying out every date.
I mean its been 5537 days so thats what, a bit over 15 and a half hours? 2 days full-time brute-forcing and done. Or like 2 minutes if you automate the task...