Yeah, just use btcrecover and set up an appropriate tokens file. Even with a fairly modest GPU, you should be able to brute force 1,000 passwords a second or more for a Core wallet file.
First see if you have any other old documents from your father in which he wrote the date - letters, spreadsheets, emails, etc. - and see what format he used. If he used that format for the password, then btcrecover could try every date in the last 100 years in about 30 seconds. Failing that, then you would need to try every other possible date format. This includes two or four digit years, digits or written words for the month, including short and full names (Jan/January), as well as every possible ordering, and every possible separator (/ - . etc.) But even then, you should be able to cycle through all that for every date in the last 100 years in a couple of hours.
If there is something else in the password, such as a time, or a location, or a name, then things become exponentially more complicated.
Hello, I have a question only out of curiosity. Let's say someone could brute force. How would they go about trying the passwords ? I mean, I could for instance generate all possible dates using brute force and store them in a file. But how would I try them all ?
That's a general question that I have since I was a CS student (apparently not a great student though haha).
