Extend Expiration Dates For Renewed Memberships in Paid Memberships Pro

gistfile1.php

/*
	Important note! This code is included in PMPro versions 1.5.7 and later. 
	Adding this code will add 2x the number of days to the end of the subscription.
*/
//if checking out for the same level, add remaining days to the enddate
function my_pmpro_checkout_level($level)
{		
	global $pmpro_msg, $pmpro_msgt;

	//does this level expire? are they an existing user of this level?
	if($level->expiration_number && pmpro_hasMembershipLevel($level->id))
	{
		//get the current enddate of their membership
		global $current_user;
		$expiration_date = $current_user->membership_level->enddate;
		
		//calculate days left
		$todays_date = time();
		$time_left = $expiration_date - $todays_date;
		
		//time left?
		if($time_left > 0)
		{
			//convert to days and add to the expiration date (assumes expiration was 1 year)
			$days_left = floor($time_left/(60*60*24));
			
			//figure out days based on period
			if($level->expiration_period == "Day")
				$total_days = $days_left + $level->expiration_number;
			elseif($level->expiration_period == "Week")
				$total_days = $days_left + $level->expiration_number * 7;
			elseif($level->expiration_period == "Month")
				$total_days = $days_left + $level->expiration_number * 30;
			elseif($level->expiration_period == "Year")
				$total_days = $days_left + $level->expiration_number * 365;
			
			//update number and period
			$level->expiration_number = $total_days;
			$level->expiration_period = "Day";
		}
	}
	
	return $level;
}
add_filter("pmpro_checkout_level", "my_pmpro_checkout_level");

This code is built into PMPro versions 1.5.7 and greater. So there is no need to add this code to your setup. If you do, it will actually add 2x the # of days to the end date.