You cannot just put a jscript date object into a date field on a form.  You need to pass a numeric value, dateVal represents the number of milliseconds in Coordinated Universal Time between the specified date and midnight January 1, 1970 to the control.
 
function sendtocrm1()

{

var today1 = new Date();

var MyDateMS = Date.UTC(today1.getYear(),today1.getMonth(),today1.getDate());

window.top.opener.Xrm.Page.getAttribute("new_birthdate").setValue(MyDateMS);

}

Best Regards

Dynamics CRM Team