Formula help for decreasing carry over hours

[CharterJP]

Greetings Excel'rs

I am in need of some help.
Attached you'll find an excel sample sheet of my problem. I have employees who Accrue 12 hours of Paid time off each month. Some or all have Carryovers from the previous year that they did not use. I need the balance column to reflect the hours taken from the hours accrued per month + the carryover.

Now here is where it gets hard for me. How can I first decrease the carryover balance by the amount used and once that amount reaches zero, start decrease the current year's accrued hours. Also, for employees that have taken more time than they had accrued, they have a negative carry over amount. So I need the YTD to decrease the amount of the negative carry over balance until zero.

Thank you for your help, if I need to clear anything up, please ask and I will do my best

Cheers,

JP