If you’ve ever had the date on a cell phone or computer mysteriously switch to December 31, 1969, you may have thought it was simply random. But the reason behind this odd glitch is a nice little tidbit of computer trivia.

Unix is a computer operating system that, in one form or another, is used on most servers, workstations, and mobile devices. It was launched in November 1971 and, after some teething problems, the “epoch date” was set to the beginning of the decade, January 1, 1970. What this means is that time began for Unix at midnight on January 1, 1970 GMT. Time measurement units are counted from the epoch so that the date and time of events can be specified without question. If a time stamp is somehow reset to 0, the clock will display January 1, 1970.