time string (hh:mm:ss) to seconds and seconds to hh:mm:ss functions

conversion.js

String.prototype.toSeconds = function () { 
  if (!this) return null; 
  var hms = this.split(':'); 
  return (+hms[0]) * 60 * 60 + (+hms[1]) * 60 + (+hms[2] || 0); 
}

String.prototype.toHHMMSS = function () {
    if (!this) return null; 
    var sec_num = parseInt(this, 10);
    var hours   = Math.floor(sec_num / 3600);
    var minutes = Math.floor((sec_num - (hours * 3600)) / 60);
    var seconds = sec_num - (hours * 3600) - (minutes * 60);

    if (hours   < 10) {hours   = "0"+hours;}
    if (minutes < 10) {minutes = "0"+minutes;}
    if (seconds < 10) {seconds = "0"+seconds;}
    return hours+':'+minutes+':'+seconds;
}