Well, if it's just the number of month changes between the two dates, then that's easy - you just extract the part of the string with the month in it and do a difference.
If you want the actual time that has passed between the two dates, try using
mktime( int hour, int minute, int second, int month, int day, int year )
on each to get the UNIX timestamp in seconds, then get the difference in seconds and do the maths on that.
As months aren't actually a set size, you can't really say something like "4.8 months have passed", but as I said at the top, you can get the integer months passed by extracting the month part of the string.
