Issues with Adding 1 month to a Unix time stamp - php

Question

For some reason, I cannot get strtotime('+1 month) to work. Here is my code;
$Date = $_REQUEST['date']; //This is a unix time stamp
$Start = $_REQUEST['start']; //This is a unix time stamp
$End = $_REQUEST['end']; //This is a unix time stamp
to add a month onto my dates;
$monStart =strtotime('+1 month', $Start);
$monEnd =strtotime('+1 month', $End);
$monDate =strtotime('+1 month', $Date);
then to show my changed dates;
$vEnd = date('m/d/Y', $monEnd);
$vStart = date('m/d/Y', $monStart);
$vDate = date('m/d/Y', $monDate);
The problem that I have is that the supplied dates;
$Date = 1/31/2013
$Start = 1/01/2013
$End = 1/31/2013
Return;
$vDate = 3/03/2013
$vStart = 2/01/2013 //Only correct one
$vEnd = 3/03/2013
Please can someone help me?

Answer 1

It's jumping to March because today is 31sth Jan, and adding a month gives 31st Feb, which doesn't exist, so it's moving to the next valid date. This is a PHP bug. You can get more info on that at https://bugs.php.net/bug.php?id=44073
You can try with DateTime to over come this scenario. You can use this function for your requirement
function add_month($date_value, $months, $format = 'm/d/Y') {
$date = new DateTime($date_value);
$start_day = $date->format('j');
$date->modify("+{$months} month");
$end_day = $date->format('j');
if ($start_day != $end_day)
$date->modify('last day of last month');
return $date->format($format);
}
Now you can call :
$vEnd = add_month($monEnd, 1);
$vStart = add_month($monStart, 1);
$vDate = add_month($monDate, 1);
This will give you :
$vDate = '02/28/2013';
$vStart = '02/01/2013';
$vEnd = '02/28/2013';
Hope this helps you :)

Answer 2

DateTime is much better for handling date math as it account for things like days in the month:
$dt = new DateTime('2013-02-01');
$dt->modify('+1 month');
echo $dt->format('Y-m-d');
See it in action
Since you're using timestamps it might look like this:
$dt = new DateTime('#'.$_REQUEST['start']);
$dt->modify('+1 month');
echo $dt->format('m/d/Y');