If you click on edit (little pencil icon) and select 'Full Edit'. Now you can change the start date/time of the workout.
My Swim had problem with the first workout I've done. The time was off by an hour and the file created was named '19891231-003725-1-1499-ANTFS-4-0.FIT', which would mean that it was created in 1989! I only had the watch on for about 30mins (see time in file name 00:37:25).
All was fine with the second workout.
Thanks a lot - saved me loads of frustration !
The date was fine today though... I wonder if it gets it wrong if you don't pair it before swimming ????