public function time_slot_time() {
    $startTimeInterval = '15';
    $open = '2017-08-23T10:00:00-05:00';
    $close = '2017-08-23T19:00:00-05:00';
    // Logic
}

1:00PM - 1:15PM
1:15PM - 1:30PM
1:30PM - 1:45PM
1:45PM - 2:00PM
2:00PM - 2:15PM
...
3:15PM - 3:30PM
3:30PM - 4:45PM