Star Trek

I'm a big believer in "stardates are nonsense, and should remain nonsense," but there were efforts made to standardize them in the '90s. They weren't particularly consistent efforts, though. The full history can be found here.

In early TNG, this was the explanation:

A stardate is a five-digit number followed by a decimal point and one more digit. Example: "41254.7." The first two digits of the stardate are always "41." The 4 stands for 24th century, the 1 indicates first season. The additional three leading digits will progress unevenly during the course of the season from 000 to 999. The digit following the decimal point is generally regarded as a day counter.

By TNG season 6, they were going with:

A Stardate is a five-digit number followed by a decimal point and one more digit. Example: "46254.7". The first two digits of the Stardate are "46." The 4 stands for the 24th Century, the 6 indicates sixth season. The following three digits will progress consecutively during the course of the season from 000 to 999. The digit following the decimal point counts tenths of a day. Stardate 45254.4, therefore, represents the noon hour on the 254th "day" of the fifth season. Because Stardates in the 24th Century are based on a complex mathematical formula, a precise correlation to Earth-based dating systems is not possible.