Something alike:
def calculate_dates
self.openedon = Date.today
case self.noy
when '1'
self.md = self.openedon+1.year
when '2'
self.md = self.openedon+2.years
when '3'
self.md = self.openedon+3.years
when '4'
self.md = self.openedon+4.years
when '5'
self.md = self.openedon+5.years
else
self.md = self.openedon+6.months
end
end