Pro-rating Rent for daily figure

[Dwight Rigel]

I picked up a SFR with a tenant in place. After meeting and discussing the new rent amount and late fees they asked if they could move their due date back. I agreed and told them that the first month would have and added prorated amount. My question is do you simply take the monthly rent multiply it times 12 and then divided by 365 for the daily figure or what formula do you use when prorating the rent for a specific number of days?