The database I manage has 3 different week formats in it for reporting periods. Sun-Sat, Fri-Sun and Mon-Sun. We also have 5 different definitions of "year" depending on fiscal year, calendar year, operational year, and the operational years for 3 different large clients, etc.

As I've slid into adulthood the wisdom of Douglas Adams has really begun to resonate: "time is an illusion, lunchtime doubley so"