comparing a date field with a fixed date in variable

Hi,
can someone please help me with the following:

I want to say:

=If [In Production Date] >= "2015-12-14" Then "Closed" Else "Open"

But, I keep getting this error: The expression or sub-expression at position 28 in the '>=' function uses an invalid data type. (IES 10037).
Any idea how to fixit?

Thanks,