We use a Prorata billing system.
Our prorata date is the 28 of each month. After this day of the month has been reached, a client will also be charged for the next month in their initial payment.
If the client signed up on the 27th of the month, they'd only be charged from the 27th to the end of the month. They will then be billed the full amount on the 1st of the month. If the client signs up on 28th or after, they will be charge the pro-rated amount + next month in advance.