In most cases, the date rent is due is determined by the clause set out in your tenancy agreement and is often the same day as when the tenancy started. For example, if the tenancy started on a Friday 13th, the rent would be due on the 13th of each month. In some cases, however, especially when falling in line with a payday, it's better to move the date rent is paid. This is possible through something called 'apportioning the rent', and it works like this:
If the rent was £650 a month, due on the 13th of the month, and you wanted it changing to the 23rd of each month, the difference of this would be 9 days. The calculation would be:
(£650.00 x 12 / 365)*9. This calculates the daily rent (because there are different number of days in different months) and times it by the number of days required.
Future rents would then be made payable on the 23rd of each month, but it's important to note that the length of the tenancy would still remain the same, and in the case of the example, would still end of the 12th of the month. Therefore the final month's rent would only be for the remaining days calculated as £650 - 192.33.
One final thing to consider is if the tenancy rolled onto a periodic tenancy, the notice period you would have to give may still run from the original date.