^ If you're writing a story in the TOS era, just use whatever as long as it's four digits and one passed the decimal point.
If you're setting your story in the TNG era, you have
to get the first two digits right. A story set during TNG season 1 (in 2364) should be stardate 41xxx.x. If it's set during TNG season 7 (in 2370) it should be 47xxx.x.