My new seamaster arrived today

WatchTimes

From the Omega website

"OMEGA uses the quick date-change system, which starts to change the date at about 11.30 pm and comes to its end at 00.10 am. The date of the ending day can be seen in full until 11.50 pm. Between a period of 10 minutes before and after midnight, the calendar jumps and lets appear the date of the next day"

So if it really bothers you take it to an AD or send it in to Omega and see what they say.

If it only gained 10 seconds over 5 days that is not a big deal "mean daily rate" test for which a standard men's watch size mechanical movement, the watch must maintain an accuracy within -4 to +6 seconds of variation per day (that's +99.994% accuracy!).

Archer

There are three different types of date change mechanisms in use generally:

1 - Slow date change - in this type the date creeps very slowly through the date window at a constant speed, and this can take several hours to complete.

2 - Semi-instantaneous date change - this is the system in your watch. The date begins to move in the window at around 11:30, and when the date is part way through the change, the tooth of the date indicator reaches a point where it slips down the sloped part of a spring (date jumper) and it snaps in place to the next full date. Tolerances for the completion of the date change are +/- 10 minutes of midnight, which is a generous tolerance. With even a little care, it can easily be made to change within a few minutes either way. Because of the way this system is designed, it can have some variation in when the date changes.

3 - Instantaneous date change - this is the style you are used to on your Rolex. The date just snaps instantly from one date to the next - of course there is a long period where a spring is loaded before this happens, but the parts you see happens instantly.

Cheers, Al