JavaScript – Input a date then autopopulate next field with difference

user1318430

Hi this is making me nuts. I am not a developer. Trying to get this to work.
User puts in a date (hire date) into form using Date object (calendar)
Next field should take that date and subtract todays date to get the length of employment.
But I get undefined in the field AND my original hire date disappears.
Here is what I have, help please, much appreciation!

//grab date of hire
try{document.getElementById("dData_DOH").onchange =          custom_calculateDate;}catch(e){}
//not sure if necessary - field that the difference should go to
try{document.getElementById("dData_LengthEmp").onblur =     insertDate;}catch(e){}

//Function to grab input hire date 
//Create variable for now
//Create variable for difference


function custom_calculateDate(){
 var hireDate = document.getElementById("dData_DOH").value = "";
     var timeNow= new Date();
 var diff = Math.abs(timeNow - hireDate);
document.getElementById("dData_DOH").setAttribute('value',     custom_calculateDate());
     }

//Function to get the difference into the LengthEmp field    
function insertDate() {
document.getElementById("dData_LengthEmp").setAttribute("",     custom_calculateDate());
}

I know this is completely wrong, as I said I am not a developer or programmer, I cannot figure out how to get this information into this field and get my original field to still show.

Thank you for reading this!