Understanding Man Days
Man days comprise of:
i) Man days - Possible shifts
ii) Man days - Actual worked shifts
iii) Man days lost:
a) Employees deployed on other duties
b) Employees availed leave > approved & unauthorized, etc.
c) Due to labor unrest like strikes, dharna, hartal, bandh, lockout, lay-off, etc.
When you project under these classifications, the comparison with actuals should tally with possible shifts. Employees who left the company cannot be accounted for available shifts or for lost shifts.
Here, shifts equal man days if the unit works on a regular/single-shift basis. If they are working on 2 shifts (A & B Shifts) and 3 shifts (AB & C shifts), then all put together summed up are man days.
For this purpose, you should have the "daily manpower on duty"; rosters/roll calls should be kept updated always. This is possible only when daily attendance is recorded accurately. This process is part of HR/MIS reporting.