I had a weird one with my Garmin Forerunner 235 this morning. Checked the time before I went out, and it said 5:17, but it was 8:29. So while I warmed up (5 minute outside walk) I watched to see when it connected to the GPS, and it didn't! Then it said the time was 11:29....
It was misty, and a very odd thick sort of mist too, started about 30m above the ground. My theory is that the mist was thick enough that the watch couldn't get a good signal from any satellite, so guessed.
About 10 minutes into my run, it suddenly bleeped, and was reading the right time. Oddly the GPS must have been working a bit, because it got my route and pace spot on, even those first 10 minutes.
Don't you just love technology?