I just had this happen to one of my non-activated phones. I logged into ABT earlier that day and everything was fine, and then again in the evening, I got the time travel warning. I am not sure, but I think I lost around 15-20 levels.
I learned a long time ago that playing with the phone’s time is like playing with fire with this game. So I never change the time. I believe the problem was this phone was in airplane mode, with WiFi on, and the time wasn’t updating.
When I turned off airplane mode, the clock increased by one lousy minute. So having a clock of by just one minute can cause this!
Usually when this happens, I will just restore from backup. Even if I loose a few days progress it is no where near the weeks of progress I lost in the time travel event. In this case, the phone’s storage was too full, and the backups are several weeks old.
I am currently looking for an app that will keep the time up-to-date via WiFi with an NTP server.
Hope this helps others!