How To Insert Colon Automatically After Entering Hours Format

I had a requirement in javascript.If I enter time as 12 it should automatically insert colon after hours and shoud allow user to enter minutes. First enter hours 12 while we are ready to insert minutes it should automatically insert colon as well as minutes.

function autoTabTimes(input, len) {
if (input.value.length == 2) {
if (input.value.indexOf(":") == -1) {
input.value = input.value + ":";
} var str = input.value.split(":");
if (str[0].length == 1) {
input.value = "0" + str[0];
}}
if (input.value.length >= len) {
input.value = input.value.slice(0, len);
input.form[(getIndex(input) + 1)].focus();
} return true;
}

The above code is working but it is not allowing user to edit the time back space is not working.