JSFiddle - React, Tailwind, and code Playground

CSS

body {
  white-space: pre;
}

JavaScript

document.body.textContent =
		foo('12:20', '01:25') + '\n'+
	  foo('23:40', '01:25')

function foo(start, duration) {
    let s = start.split(':'),
    d = duration.split(':'),
    min = +s[0] * 60 + +s[1] + +d[0] * 60 + +d[1],
    h = Math.floor(min / 60),
    m = min % 60
    return pad(h) + ':' + pad(m)
}

function pad(n) {
	return n > 9 ? n : '0'+n
}