Calculating Employee Leave Accrual

I like the idea of a table and this does work for the application. Only thing I see, and I probably didn't explain it well, at 12 months the leave earned from 6-12 months (24 Days) does not get added to the 30 days they earn once they hit 12 months.

Explanation:
tenure is 10 months I have 20 days available
tenure is 12 months, I have 24 days available
Tenure is 13 months, I have 30 days available (not 54)
This is assuming that I have not taken any time off since being hired...