codice:
<!doctype html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Esempio</title>
<script type="text/javascript">
function checkTime (oToCheck, oPssEvt1) {
	var	oKeyEvent = oPssEvt1 || window.event || { charCode: 0, keyCode: 0 },
		nChar = oKeyEvent.charCode,
		nKey = oKeyEvent.keyCode,
		sVal = oToCheck.value,
		sNewVal = sVal.substring(0, oToCheck.selectionStart - (nKey === 8 && nChar === 0 ? 1 : 0)) + (nChar > 0 ? String.fromCharCode(nChar) : "") + sVal.slice(oToCheck.selectionEnd + (nKey === 46 && nChar === 0 ? 1 : 0)), bFormat = nChar === 0 || /^\d{1,2}(?:\:\d{1,2})?$/.test(sNewVal);

	if (bFormat) {
		var bCorrect = sNewVal.split(":");
		for (var nDigit = 0; nDigit < bCorrect.length; nDigit++) {
			if (parseFloat(bCorrect[nDigit]) > 59) {
				bFormat = false; break;
			}
		}

	}

	return bFormat;
}

function formatTime (oToFix, oPssEvt2) {
	var nKey = (oPssEvt2 || window.event || { keyCode: 8 }).keyCode, nSelS = oToFix.selectionStart, nSelE = oToFix.selectionEnd, sVal = oToFix.value;
	if (nSelS === nSelE && nSelS === sVal.length && /^\d{1,2}$/.test(sVal) && parseFloat(sVal) > 5 && nKey !== 8 && nKey !== 46 && (nKey < 32 || nKey > 41)) {
		sVal += ":";
		oToFix.value = sVal.replace(/^(\d)\:/, "0$1:");
		oToFix.focus();
	} else if (/\:\d$/.test(sVal) && parseFloat(/\d$/.exec(sVal)) > 5) {
		oToFix.value = sVal.replace(/\d$/, "0$&");
		oToFix.focus();
	}
}
</script>
</head>

<body>
<form name="myForm">


<input type="text" name="myInput" onkeypress="return checkTime(this,event);" onkeyup="formatTime(this,event);" onpaste="return false;" /></p>
</form>
</body>
</html>