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<?php
function schedule_my_popup( $is_loadable, $popup_id ) {
	if( $popup_id == 123 ) {
		
		$start = strtotime( 'May 4, 2015 11:59PM' );
		$end = strtotime( 'May 5, 2015 11:59PM' );
		$now = strtotime( 'now' );

		if( $now >= $start && $now <= $end ) {
			return true;
		}
		else {
			return false;
		}

	}
	return $is_loadable;
}
add_filter( 'popmake_popup_is_loadable', 'schedule_my_popup', 1000 );