You attempted to invoke the Date.prototype.toString or Date.prototype.valueOf method on an object of a type other than Date. The object of this type of invocation must be of type Date. For example:

var o = new Object;
o.f = Date.prototype.toString;
o.f();

To correct this error

Only invoke the Date.prototype.toString or Date.prototype.valueOf methods on objects of type Date.

See Also

Date Object | getDate Method | Intrinsic Objects