(Not as exciting as you might think.)
My phone seems to be using a stockpile of old timestamps on my outgoing messages. On Saturday afternoon I sent seven messages within a few minutes. The first four were a minute or two apart up to 14:42 20/02; the next three were dated 13:41 26/01, 16:33 26/01 and 08:21 27/01. Later on Saturday two messages sent within minutes of each other were dated 12:43 27/01 (inaccurate) and 16:30 20/02 (accurate). Only one of the next twelve messages was correctly dated. Messages sent minutes apart had time gaps of five to seventeen hours. The incorrect dates have been applied in chronological order, progressing from 13:41 26/01 to 21:21 30/01 over the last forty-eight hours. The timestamps on incoming messages are okay, and as far as I know the clock has been correct.