How to add random minutes like first slots should be 30 minutes and then 1 of 15 minute slot and again 30 minute slot

Question

const eventClick = (args) => {
    let StartTime = moment(args?.event?.StartTime, 'HH:mm ')
    let EndTime = moment(args?.event?.EndTime, 'HH:mm ')
    if (EndTime.isBefore(StartTime)) {
        EndTime.add(1, 'day')
    }
    let arr = []
    while (StartTime <= EndTime) {
        arr.push(new moment(StartTime).format('HH:mm'))
        StartTime.add(30, 'minutes')
    }
    setSlotEvent(arr)
    setOpen(true)
    setCellEvent(args.event)
}
{
                            slotEvent && slotEvent?.map((data, index) => (
                                <>
                                    <div className='slotGrid'>
                                                {data}
                                    </div>
                                    
                                </>
                            ))
                        }

How to add random minutes like first slots should be 30 minutes and then 1 of 15 minute slot and again 30 minute slot