JSFiddle - React, Tailwind, and code Playground

JavaScript

var selectedTime = '11:30 PM'
var startTime = '12:00 AM'
var endTime = '13:00 AM'

function changeTime(time){
var thisTime;
var thisHour = +time.substring(0,2);
  if(time.substring(6,8) == 'PM'){
  //add 12 hours to make it 24 hour clock
  thisHour +=12;
  }
  
  return thisHour+time.substring(2,5)
  }
var selectedTimeSeconds = changeTime(selectedTime);
var startTimeSeconds =  changeTime(startTime);
var endTimeSeconds =  changeTime(endTime);

var selectedTimeSecondsParsed = hmsToSecondsOnly(selectedTimeSeconds) //pass to convert to seconds function
var startTimeSecondsParsed = hmsToSecondsOnly(startTimeSeconds)
var endTimeSecondssParsed = hmsToSecondsOnly(endTimeSeconds)

console.log(selectedTimeSecondsParsed)
console.log(startTimeSecondsParsed)
console.log(endTimeSecondssParsed)

  /* need logic to convert time to Date Format */

  if (selectedTimeSecondsParsed >= startTimeSecondsParsed && selectedTimeSecondsParsed <= endTimeSecondssParsed) { //if its between
   alert("Time in beween interval");
  } else {
   alert("Time is not with in the time Slot");
  }

function hmsToSecondsOnly(str) {
  var p = str.split(':'),
    s = 0,
    m = 1;

  while (p.length > 0) {
    s += m * parseInt(p.pop(), 10);
    m *= 60;
  }

  return s;
}