I need to calulate the following - but don't know the best/easiest formula to
use.

tst month
1100 oct
1100 nov
1100 dec
1100 oct
1100 nov
1100 dec
1100 oct
1100 feb
1100 mar
2200 mar
2200 mar
2200 jan
2200 jan
2200 feb

the result would be

1100 oct 3
1100 nov 2
1100 dec 2
1100 feb 1
1100 mar 1
2200 mar 2
2200 jan 2
2200 feb 1