+1 měsíc php
$time = strtotime("2010.12.11");
$final = date("Y-m-d", strtotime("+1 month", $time));
odečíst některé dny php
// First date of the month.
$monthFirstDate = date('Y-m-01', strtotime('today'));
// Last date of the month.
$monthLastDate = date('Y-m-t', strtotime('today'));
// Second last date of the month
$monthLastSecondDate = date('Y-m-d', strtotime('-2 day', strtotime('today')));