28th of October, 1997

Do you remember what you what you were doing that day? perhaps you weren't even alive. I'm not trying to solve any murders or bizarre hit and run incidents but someone recently gave me a Garmin GPS 12 which had been in a drawer for quite some time.

I fired 'er up anyway after a bit of tinkering and it got a fix soon enough, only problem is that it's convinced today is 28/10/1997. The reason for the incorrect date seems to be quirk in the protocol of GPS itself. A sort of millenium bug of some sort though if the device had been in regular use and the small internal coin cell hadn't run out it wouldn't have ever cropped up

As opposed to the year, month, and day format of the Gregorian calendar, the GPS date is expressed as a week number and a seconds-into-week number. The week number is transmitted as a ten-bit field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024 weeks (19.6 years). GPS week zero started at 00:00:00 UTC (00:00:19 TAI) on January 6, 1980, and the week number became zero again for the first time at 23:59:47 UTC on August 21, 1999 (00:00:19 TAI on August 22, 1999). To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584 days) to correctly translate the GPS date signal. To address this concern the modernized GPS navigation message uses a 13-bit field that only repeats every 8,192 weeks (157 years), thus lasting until the year 2137 (157 years after GPS week zero).