Digital signature

I am trying to get my document to save as a new name once signed or even put that information on signature once signed.  I used the above script like this:

var currentTime = new Date()

    var month = currentTime.getMonth() + 1

    var day = currentTime.getDate()

    var year = currentTime.getFullYear()

    var signingTime = month +"/"+day+"/"+year  //

    var f = this.getField("SOP No+Version No");  //

    f.value = signingTime;

I could not get it to work????