Technically it wouldn’t be a stardate. Remember in ENT each log was called a “star log”. It contained a Gregorian date format. Stardates didn’t exist till the beginning of the Federation (the existence of non-Federation using stardates like Romulans in Kelevin Timeline withstanding). So if you tried calculating a stardate before 2161, you’d get a negative number.
EDIT: Yes you can convert dates but depending on the century the formula changes. The Sisko Day and Captain Picard Day are the only days I know of that fans agree upon a Gregorian date.
Technically it wouldn’t be a stardate. Remember in ENT each log was called a “star log”. It contained a Gregorian date format. Stardates didn’t exist till the beginning of the Federation (the existence of non-Federation using stardates like Romulans in Kelevin Timeline withstanding). So if you tried calculating a stardate before 2161, you’d get a negative number.
EDIT: Yes you can convert dates but depending on the century the formula changes. The Sisko Day and Captain Picard Day are the only days I know of that fans agree upon a Gregorian date.