Where I live when you move in mid month they usually charge you your security deposit and a full month's rent at move in and then on the first of the next month instead of paying for another full month's rent you just pay a prorated amount based on how many days you lived there in the first month.

It seems counterintuitive since you pay at the beginning of the month it would logically make sense to pay a prorated amount at move in and a full month on the first of the next month. But they do it that way so they have a little more money for the first few weeks just in case you turn out to be a deadbeat.