How to assign date vaşue to entity attribute using xrmservice toolkit library in crm 2015?

Hi,

On my html web resource i have an calendar control. I select a date.

I put the date to a variable as  

 var dateVal= document.getElementById("test").value;

and i want this value to assign to the entity attribute. But i can't the xrmservice library gives me error. Her is my assigment :

  entityObj.attributes["new_billdate"] =dateVal;  

How can i  achieve this? Cn anyone help me please?