Mortgage calculator that can figure payment period if extra money is applied directly to principle each month?

My fiancee and I are trying to figure out how much time (and money) would be shaved off of our mortgage repayment period if we have extra money applied directly to the principle each month, but none of the mortgage calculators I’ve seen online specifically have a space where a particular dollar amount can be [...] http://bit.ly/jlvW3B