Originally Posted by coffee_monster
There are for those sorts of things. The thing that allows those calculators to exist is consistent payments, both in amount and time. If you want to look those calculators up, look for 'annuity' calculators.
In OP's situation, we have random amounts of repayments at random times. That's the difference between his situation and a mortgage payment.
It's not all that hard. But to make it easier on op, I've created a sheet:
<-- wrong, this is better
The basics are calculating interest based on
(previous balance) * 1.1 ^ ((days passed since last change)/365) - (previous balance)
for each interest accrual during that period.
Add up those and you're there.
Just check out the sheet.