Countdown

6 clues
The Final ___
NASA arithmetic
Activity preceding liftoff
Pre-midnight numerical ritual
Mark the transition from an old year to the new, maybe
What might end the year, and a hint to five answers in this puzzle
Similar answers:
Account
Accountable
Accountant
Accountants
Accountantshope
Accountbooks
Accountexecutive
Accountholder
Accountingclerk
Accountingcrows
Accountingerror
Accountingsheep
Accountofrace
Accounts
Accountsfor
Accountspayable
Accountsplayable
Acountpastdue
Adcouncil
Advisorycouncil