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.
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.
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.