Aqui te lo explican mas detallado.. espero le entiendas cualquier ocas me avisas..
http://www.webestilo.com/foros/mensa...o=11&msg=39495
tambien puedes revisar las FAQS del foro.. aqui esta este codigo.. espero te sirva..!
(Kaopectate creador)
Aqui..
<html>
<head>
<script language="JavaScript">
var aFinMes = new Array(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);
function finMes(nMes, nAno){
return aFinMes[nMes - 1] + (((nMes == 2) && (nAno % 4) == 0)? 1: 0);
}
function padNmb(nStr, nLen, sChr){
var sRes = String(nStr);
for (var i = 0; i < nLen - String(nStr).length; i++)
sRes = sChr + sRes;
return sRes;
}
function makeDateFormat(nDay, nMonth, nYear){
var sRes;
sRes = padNmb(nDay, 2, "0") + "/" + padNmb(nMonth, 2, "0") + "/" + padNmb(nYear, 4, "0");
return sRes;
}
function incDate(sFec0){
var nDia = parseInt(sFec0.substr(0, 2), 10);
var nMes = parseInt(sFec0.substr(3, 2), 10);
var nAno = parseInt(sFec0.substr(6, 4), 10);
nDia += 1;
if (nDia > finMes(nMes, nAno)){
nDia = 1;
nMes += 1;
if (nMes == 13){
nMes = 1;
nAno += 1;
}
}
return makeDateFormat(nDia, nMes, nAno);
}
function decDate(sFec0){
var nDia = Number(sFec0.substr(0, 2));
var nMes = Number(sFec0.substr(3, 2));
var nAno = Number(sFec0.substr(6, 4));
nDia -= 1;
if (nDia == 0){
nMes -= 1;
if (nMes == 0){
nMes = 12;
nAno -= 1;
}
nDia = finMes(nMes, nAno);
}
return makeDateFormat(nDia, nMes, nAno);
}
function addToDate(sFec0, sInc){
var nInc = Math.abs(parseInt(sInc));
var sRes = sFec0;
if (parseInt(sInc) >= 0)
for (var i = 0; i < nInc; i++) sRes = incDate(sRes);
else
for (var i = 0; i < nInc; i++) sRes = decDate(sRes);
return sRes;
}
function recalcF1(){
with (document.formulario){
fecha1.value = addToDate(fecha0.value, increm.value);
}
}
</script>
</head>
<body>
<form name="formulario">
<table>
<tr>
<td align="right">
Fecha (dd/mm/aaaa):
</td>
<td>
<input type="text" name="fecha0" size="10">
</td>
</tr>
<tr>
<td align="right">
Incremento:
</td>
<td>
<input type="text" name="increm" size="3">
</td>
</tr>
<tr>
<td align="right">
Resultado (dd/mm/aaaa):
</td>
<td>
<input type="text" name="fecha1" disabled size="10">
</td>
</tr>
<tr>
<td colspan="2" align="center">
<input type="button" onclick="recalcF1()" value="Calcular">
</td>
</tr>
</table>
</form>
</body>
</html>
SALU2..!!