timeintervals.js

/**
 * Little script/function to get time intervals
 */

// Based on: 
// https://stackoverflow.com/a/19225446/808870
const diffInHours =  (startDate, endDate) => ( endDate.valueOf() - startDate.valueOf() ) / 1000 / 60 / 60;

const getTimeIntervalsPerHour = interval => {
    var times = [];
    var i =0;
    while (i < 60) {
        times.push(i);
        i = i + interval;
    }
    return times;
}

const getAvailableTimeSlots = (startTime, endTime, minuteInterval) => {
    var availableTimeSlots = [];
    var currentIntervalTime = startTime;
    while (currentIntervalTime < endTime) {
        var minuteIntervalInMilliseconds = minuteInterval * 60 * 1000;
        availableTimeSlots.push(currentIntervalTime);
        currentIntervalTime = new Date(currentIntervalTime.getTime() + minuteIntervalInMilliseconds);
    }
    return availableTimeSlots;
}

const startTime = new Date('2020-02-19 11:00 EST');
const endTime = new Date('2020-02-19 21:00 EST');
const minuteInterval = 15;
const numberOfHoursOpen = diffInHours(startTime, endTime);


console.log(getAvailableTimeSlots(startTime, endTime, minuteInterval));