new-duedate-conversion.php

$originalStartDate = app('Carbon\Carbon')->parse('1/30/2017');

$newStartDate = app('Carbon\Carbon')->parse('2/27/2017');

$difference = $originalStartDate->diffInDays($newStartDate);

$dates = [
    '2/20/2017',
    '2/24/2017',
    '3/6/2017',
    '3/27/2017',
    '4/3/2017',
    '4/10/2017',        
];

foreach($dates as $key => $date)
{
    $dates[$key]['original'] = app('Carbon\Carbon')->parse($date);
    $dates[$key]['new'] = app('Carbon\Carbon')->parse($date)->addDays($difference);
}

foreach($dates as $key => $date)
{        
    echo "original: " . $date['original']->format('l m/d/Y') . "<br>";
    echo "new: " . $date['new']->format('l m/d/Y') . "<br>";
}