How to change month data value based on hemisphere in Excel -

i examining how particular variable in biology affected differences in season. quantatize data, used datevalue function in excel convert month number. however, have data both northern , southern hemispheres, , want set equation month data southern hemisphere staggered 6 months. is, january counted "1" if data point northern hemisphere , "7" if south, , on.

i having trouble finding way enter formula in excel take account. code have come far

=if(a1="northern",month(datevalue(b1&"1")),month(datevalue(b2&"7")))

where a1 hemisphere , b1 month. know how set up?

this should after:

=if(a1="northern",month(eomonth(b1,0)),month(eomonth(b1,6))) 

you don't need first eomonth there left in in case ever want add months northern hemisphere