Nepali calendar/birthdates?

I swear, never a dull moment in this job. I have in front of me a university diploma from Kathmandu, listing my client’s birthdate as 2023/02/04, and that he attended university from 2045 to 2047.

So unless he’s really ahead of his time, Nepal has a different calendar. I poked around a bit on Google and found lots of sites that would tell me when various holidays fall on the Nepali calendar, but that’s boring and no fun. Anyone happen to know a bit about the origins of the Nepali calendar system? Is it a mythological or historical one? And in anticipation of getting flak from the Bureau of Citizenship and Immigration Services when we file his green card application, any official-looking calendar conversion sites?