In a new post to his site Ben Ramsey talks about why dates are hard and can be frustrating to work with sometimes. It revolves around an issue he recently found with calculating a time for a UUID and days of the week.

No, I’m not talking about a meeting with a lover or potential lover. While those can be stressful, the calendar math used to determine the precise date and time on which such a meeting might occur is infinitely more difficult to perform. To software programmers, this isn’t news, but I recently encountered an issue when calculating the time for an RFC 4122 UUID that had me questioning the accuracy of our modern, accepted calendars, especially with regard to the days of the week on which our dates fall.

In his work on his UUID library, he came across a the bug because of some failing unit tests. It was only happening in certain versions of PHP and upon further investigation found the issue to be a wrong day of the week from a date in 1582 (the correct value being Sunday). As it turned out, the date in question was actually a Saturday and his local environment was reporting bad results. The problem was with a revision made to the Gregorian calendar, removing 10 days causing a difference between the Gregorian and Great Britain versions of 1582.