Interest calculated daily and charged quarterly

Hi guys.

Hope you can help. I dont know where to begin. I can calculate on paper but it takes to long. There must be an easier way.

I have a home loan where the interest in calculated daily on the outstanding balance and charged to the account quarterly (the interest is added to the principal quarterly)

On paper i do the outstanding daily balance * interest rate = x
Then i will calculate x/360 = daily interest. Then i repeat every day for a quarter based on what is outstanding (Ie that will change once i make a payment)
Then i add them all together to work out what the total owing is.

I would like to be able to use and excell formula that will work out the interest that will be charged at the end of that quarter if i make additional payments on x day in that period. (The x day will differ sometime the extra payment will be at the start of the month and sometime it will be at the end and i would like to be able to work out if i make more than one x number of payments)

Ie I generally pay into the loan every week (but the amount is different every week)

In addition...
I know it may be difficult do you think it is possible to work out if i pay x amount into the loan per week (Amount the same) interest calculated daily charged at the end of the quarter, if it is possible to work out the time it would take to pay off the loan?

Thank you in advance.