Let’s say, for example, that a system supplies you the time of some event in UTC, you convert it to local and shove the date/time up on the display. Say, for argument’s sake, you also include the Day Of Week, ending up with a format of DD/MM, DOW HH:MM. Everything looks fine, until someone notices that the Day Of Week is wrong. The 28th of May is a Wednesday, not a Thursday.

What happened?

The date conversion routine that generates the DOW string does a bunch of odd stuff, but seems to work correctly; it certainly works in other parts of the code, and generates the right string there.

WTF?

The UTC time seemed to be converted to local time twice, but that wasn’t the culprit; surprisingly, no-one is killed in an explosion of silicon splinters when that code is double-executed. Whatever.

Could it be that the system supplying you the time of that event in UTC is off by a year? One year into the future. That would give you that behaviour.

Check it.