When you first move in we always need to take one months rental payments in advance
We also need to align the rental payments to the start of the month or as close as we can
If you moved in mid-month, say the 15th of November you will pay your first months rent in advance (say £400) and your deposit (say £400)
However we then need to align you so that you are paying on the 1st of every month, thus on the 1st December you will just pay from the 15th to the 31st November, otherwise you will be overpaying during that month, having not used all of your original month in advance payment.
Therefore on the 1st December 17 days pro-rata rent will be due
£400/31 = £12.90
£12.90 x 17 = £219.35 due 1st December
Then you are all set to start paying normally on 1st January
:-)