Help with expiration dates & sums

I'm doing time keeping at work and there is a certain type of leave that expires after one calendar year. I'm wondering if there is a way to subtract time from a total if the expiration date for that earned leave has passed.

For example - I have a total of 50 hours of leave and I earned 8 hours of this leave last September and 10 hours in November. I have not used any of those 18 hours, and since September of this year has passed, I need to subtract the September 2017 hours from my total, leaving me with 42, without subtracting the 10 hours I earned in November 2017 since it has not expired yet. 

I will attach an example spreadsheet with the formulas I am unsuccessfully trying to use.