Lynne
This should do the trick for you.
'Borrowed' from a post a month or so back
Remember they are text so no date calculations can be done without more
string manipulation


=CHOOSE(MATCH(A1,{"Jan";"Feb";"Mar";"Apr";"May";"J un";"Jul";"Aug";"Sep";"Oct";"Nov";"Dec"},0),"Apr", "May","Jun","Jul","Aug","Sep","Oct","Nov","Dec","J an","Feb","Mar")

HTH

Rob