DateRange class: Quickly find overlaps with other date ranges

DateRange.php

<?php
/**
 * Class to make it easier to compare date ranges (where range is between start date and end date)
 */
class DateRange {

	public $StartDate = null;
	public $EndDate = null;

	/**
	 * Accept string dates, Date & DateTime objects
	 * @param [mixed] $StartDate 
	 * @param [mixed] $EndDate  
	 */
	public function __construct($StartDate = null, $EndDate = null) {

		$valid_classes = array('Date','SS_DateTime');

		foreach (array('StartDate','EndDate') as $property) {
			if (empty($$property)) {
				return;
			}
			if (!is_object($$property) ) {
				$obj = DBField::create_field('Date',null);
				$obj->setValue($$property);
			} else {
				foreach ($valid_classes as $class) {
					if (get_class($$property) == $class) {
						$obj = $$property;
						break;
					}
				}
				user_error('Class is not: ' . implode(', ', $valid_classes));
			}
			$this->$property = $obj;
		}

	}

	/**
	 * Does the current daterange overlap with the provided daterange?
	 * @param [DateRange] $DateRange Provided daterange
	 */
	public function OverlapsWith(DateRange $DateRange) {
		if ( $this->StartDate <= $DateRange->EndDate && $DateRange->StartDate <= $this->EndDate ) {
			return true;
		}
		return false;
	}

}

Example: checking for overlapping Schedules

$ThisDateRange = new DateRange($this->StartDate, $this->EndDate);

foreach ($CourseDate->Schedules() as $Schedule) {
    // Don't match the current ID
    if ($Schedule->ID == $this->ID) {
        continue;
    }
    $ScheduleDateRange = new DateRange($Schedule->StartDate, $Schedule->EndDate);
    if ($ScheduleDateRange->OverlapsWith($ThisDateRange)) {
            $result->error(sprintf("Cannot save: Another schedule (#%d) overlaps with this one.", $Schedule->ID));
            break;
    }
}