Eligle Vacation Hours Based On Hire Date

I'm looking for a formula that will figure the amount vacation hours an employee is eligble for based on his/her hire date and going by the current days date. It would keep me from manually figuring an employee's vacation time that they are due every time their anniversary date rolls around.

Here are the parameters that my company uses in order to award vacation time:

1) After the 1st year of employment, full time employees get 40 hours.

2) After the 2nd year of employment, Full time employees get 80 hours.

3) After the 9th year of employment, full time employees get 120 hours.

4) After the 16th year of employment, full time employees get 160 hours