calendar error

I have a simple calendar and want it to populate a text box.

When I use this code on a blank page it works. When I use it on my form page with another button, it gives me an error message

 

 Microsoft JScript runtime error: Object doesn't support this property or method
function __doPostBack(eventTarget, eventArgument) {
if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
theForm.__EVENTTARGET.value = eventTarget;
theForm.__EVENTARGUMENT.value = eventArgument;
theForm.submit();
}
}
...