Pass the VALUE of an INPUT to an OBJECT

I have a JSP in which I have to modify an INPUT in a certain way and its final value should be passed to an object, I have tried it in several ways but I do not recover the value. I leave here some examples of how I have tried to do, if someone has any idea what is missing what I'm grateful.

Ahem 1:

<input name="txtFechaCancelacionDia" type="text"  size="2" maxlength="2" value="<%=diaC%>" onblur="ValidaDate(this.form,this)"><strong> / </strong>
                                <input name="txtFechaCancelacionMes" type="text"  size="2" maxlength="2" value="<%=mesC%>" onblur="ValidaDate(this.form,this)"><strong> / </strong>
                                <input name="txtFechaCancelacionAno" type="text"  size="4" maxlength="4" value="<%=anoC%>" onblur="ValidaDate(this.form,this)">
                                <script>
                                   var diaCM = document.getElementById("txtFechaCancelacionDia").value;
                                   var mesCM = document.getElementById("txtFechaCancelacionMes").value;
                                   var anoCM = document.getElementById("txtFechaCancelacionAno").value;
                                   var dateCancelModif = diaCM + '-' + mesCM + '-' + anoCM;
                                   <%lobjContrato.mstrFechaCancelacionModif%> =  dateCancelModif;
                                </script>

Ahem 2:

var strDayC   = frmForma.txtFechaCancelacionDia.value; var strMonthC = frmForma.txtFechaCancelacionMes.value; var strYearC  = frmForma.txtFechaCancelacionAno.value;
var strDateC = strDayC + '-' + strMonthC + '-' + strYearC;
alert(strDateC);
frmForma.txtFechaCancelacionModif.value = strDateC;
alert ('frmForma.txtFechaCancelacionModif.value: ' + frmForma.txtFechaCancelacionModif.value);