Employee count by month

Sorry if this has already been asked/answered but I couldn't find a thread on it. I have a sheet with every employee ever hired/fired and the dates. Given this master list I need a formula to calculate how many employees were active on a given month. We are trying to compare variance from a given month against the number of employees during the same month of the previous year.

ie:
January: 627 active employees, 15 separated = 2.23% turnover in 2016. We had 4.16% turnover in 2015 which is a variance of -1.93% year over year.
The problem I'm running into is I don't know how to calculate the number of employees who were active on a given month from a list that goes back to the 1980's.

The list of employees is on the "Master" tab, and the cells I'm having a problem with are on "Turnover 2016" C15:C33
NOTE: the currently attached sheet is a MUCH abbreviated version, I didn't want to bog down the forum servers.