JSFiddle - React, Tailwind, and code Playground

JavaScript

var convertTime = function(timestamp, separator) {
    var pad = function(input) {return input < 10 ? "0" + input : input;};
    var date = timestamp ? new Date(timestamp * 1000) : new Date();
    return [
        pad(date.getHours()),
        pad(date.getMinutes()),
        pad(date.getSeconds())
    ].join(typeof separator !== 'undefined' ?  separator : ':' );
}

document.body.innerHTML = '<pre>' + JSON.stringify({
    920535115 : convertTime(920535115, '/'),
    1061351153 : convertTime(1061351153, ':'),
    1435651350 : convertTime(1435651350, '-'),
    1487938926 : convertTime(1487938926),
    1555135551 : convertTime(1555135551, '.')
}, null, '\t') +  '</pre>';