Try this

= Sum(if(Text([Range]<"MMMYYYY") = "May2008",1,0))

but instead of pressing return pressing return once you have completed your
formula - you need to press Sht+Ctrl+Rtn

This will put {} brackets around the formula to show it is an array formula...

However - please note - in the range you can't put "E:E" it needs to be
defined.. so "E1:E4000"

HTH
Rob