That's rubbish. There are absolutely no time constraints (that I'm aware of) on registering newly delivered cars. Simply put the dealer wants his cash and the car sold asap so he doesn't have to store it and is spinning you a line accordingly.
It also makes his December deals (number of cars sold) look better if he registers this month.
Stick your ground for a January registration date - if he still objects, then ask him to point you to the necessary government legislation that says so. Having said that, do check the small print of the agreement you signed when you put your deposit down just in case you have inadvertantly signed up to something you were not aware of, i.e. that you will take delivery with "n" days of the car being delivered to the dealer.