[рони]

zava75,

var start = '09:00';
var arr  = start.split(":");
var mn = arr[0] * 60 + arr[1] * 1;
    mn += 30;
    arr[0] = mn / 60 | 0;
    arr[1] = mn % 60;
    arr = arr.map(num => num.toString().padStart(2, "0"))
var time = arr.join(":");
    alert(time);