months_between returns number of months between dates date1
and date2. If date1 is later than date2, then the
result is positive. If date1 is earlier than date2, then
the result is negative. If date1 and date2 are either the
same days of the month or both last days of months, then the result is
always an integer. Otherwise Oracle calculates the fractional portion of
the result based on a 31-day month and considers the difference in time
components date1 and date2.