It looks like you're looking for a method of calculating interest earned in a savings account for a specified number of days.
Interest in savings accounts are typically compounded daily. You can get pretty close with 1/360th of your interest rate times the number of days times the amount of money.
To get closer you do (using the variables you specified)
( y * ( 1 + ( x/100/360 ) ) ^z ) - y
If you are paying the statement in it's entirety, and you were not carrying a balance in to the prior statement, there will be no interest charge if you pay on the due date.
Generally, interest is calculated monthly based on your average daily balance (total balance each day divided by number of days in the billing period). If you pay your balance in full each month there won't be an interest charge. You accrue your charges through the statement period. You are then given until the statement due date to pay the balance off. If you carry a balance past that due date you will begin to accrue interest. What's worse, if you carried a balance through the due date, you will begin accruing interest on your new charges as well.
The best possible first step to debt reduction is to stop spending on any card with a balance because any new charges will begin to accrue interest immediately.
Interest rate divided by 360 will typically get you pretty close to a daily interest amount.
I personally never pay before the due date.