JSFiddle - React, Tailwind, and code Playground

JavaScript

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

var selectedTimeSeconds = selectedTime.substring(0,5) + ':00';
var startTimeSeconds = startTime.substring(0,5) + ':00';
var endTimeSeconds = endTime.substring(0,5) + ':00';

var selectedTimeSecondsParsed = hmsToSecondsOnly(selectedTimeSeconds)
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) {
   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;
}