Date validation - no empty and null input allowed, How?

I have the following JS

<script language = "Javascript">
......
function ValidateForm(){
      var dt=document.NetFlow.BegDate
        var dt2=document.NetFlow.EndDate
        var dt3= document.NetFlow.Num_Results
         
      if (isDate(dt.value)==false)
        {
            dt.focus()
            return false
         }
         else if (isDate(dt2.value)==false)
         {
                dt2.focus()
            return false
      }
       
       else if(dt3.value.match(/[^0-9]/))
        {
        alert("Invalid data format.\nOnly numbers are allowed.");
        dt3.focus()
        return false
        }
    return true
 }
 
 
 function ValidateIntlNumber(fld) {
     if (fld.value.match(/[^0-9]/)) {
     
          alert ('Only numbers are permitted');
          // Un comment the next line is you want to remove the non-numbers automatically
          fld.value=fld.value.replace(/[^0-9]/g,'');
     }
}
</script>

........

                    <label>Degree of Imbalence: <input name="DegImb" type="text" size="3" value="65" onkeyup="ValidateIntlNumber(this);"> ( enter as 50% )&nbsp; &nbsp; </label>
                    <label>Total Flows:<input name="TotalFlowImb" type="text" size="3" value="10" onkeyup="ValidateIntlNumber(this);" > ( enter as  )</label>
               
 <label># of Results: <input type="text" name="Num_Results" size="3" maxlength="3" value="25" onkeyup="ValidateIntlNumber(this);" ></label>
           
 <label>Date Range: <input type="text"  size="20" maxlength="30"  name= "BegDate" value="<%=y%>" > - <input type="text" size="20" maxlength="30" name="EndDate" value=<%=s%> > ( enter as mm/dd/yyyy)</label>
        </div>

.......

I have one question to improve my code
I don't want null value or empty value be stayed in the any fields above. That means user must input a valid data into the field, can not leave null, blank before he submits the form. How to do that?