try with this below textbox.text never give an object.
function fn_getdays()
{
var txtfrom = document.getElementById('txtfmdt').value;
var txtodt = document.getElementById('txttodt').value;
var sName1=_Default3.GetCheckdt(txtfrom,txtodt);
document.getElementById('txtnodt').value = sName1.value;
}
and
var sName1=_Default3.GetCheckdt(txtfrom,txtodt) is calling ajax method