Pass the value of a parameter to another

Hello


I have a parameter ($year, format: yyyy) which changes following a multiselect component. I would like to obtain from $year other two parameters ($start_date and $end_date, format yyyy-mm) according to certain rules:


Example:
$year= 2014
$start_date = 2014-01
$end_date = 2014-12


$year= 2015
$start_date = 2015-04
$end_date = 2014-05




How can I do it?


thanks A Lot