public static function daysToWeekStart($date, $first_day = 'monday')
{
$week_days = array('monday' => 0, 'tuesday' => 1, 'wednesday' => 2, 'thursday' => 3, 'friday' => 4, 'saturday' => 5, 'sunday' => 6);
$start_day_number = $week_days[$first_day];
$wday = $date->format("w");
$current_day_number = $wday != 0 ? $wday - 1 : 6;
return WPDKMath::rModulus($current_day_number - $start_day_number, 7);
}