Find the interesct on a dynamic graph

[Kraner]

In a few words I am riding all the roads in a two county area to raise money for cancer research and would like to calculate the 'end date'. The graph and data are below. The data will be updated at the end of each month.

I plugged the current data into a graphing calculator and got 9.48 or ~September 15th. Is there a way to do this automatically in excel?

Thanks!