Sounds liek the phone crashed and received a call before the phone could update its time stamp.
All Macs have a default time and date that it defaults to when they loose complete power or crash really hard. Usually this time/date is in the past. I assume the iPhone is similar but maybe its default is in the future?. A Mac will attempt to resync its time with a NNTP time server, usually time.apple.com. The iPhone will attempt to resync its time as well but I don't know if this is with NNTP or some other AT&T time protocol.