By using the PHP functions date and strtotime this can be accomplished.
Example
$date = '2010-03-22';
$new_date = date('Y-m-d',strtotime('+4 months',strtotime($date)));
echo $new_date; //Displays 2010-07-22
Explanation of PHP functions:
1)
FUNCTION date
DESCRIPTION string date ( string $format [, int $timestamp ] )
PARAMETERS USED IN THIS EXAMPLE
Y = A full numeric representation of a year, 4 digits (Examples: 1999 or 2003) m = Numeric representation of a month, with leading zeros (01 through 12) d = Day of the month, 2 digits with leading zeros (01 to 31)
2)
FUNCTION strtotime
DESCRIPTION int strtotime ( string $time [, int $now ] )
PARAMETERS
time The string to parse. Before PHP 5.0.0, microseconds weren’t allowed in the time, since PHP 5.0.0 they are allowed but ignored. now The timestamp which is used as a base for the calculation of relative dates.