ohh yes sorry, i see the two objects are referenced, so of course you can do that.
I checked this and also got an "PostProcessing" Error, the eventlog says:
System.ArgumentException: The parameter for the function is of an unknown type: '2003-01-01 00:00'. System.FormatException: Guid should contain 32 digits with 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx).
at System.Guid..ctor(String g)
Seem that the automatic GUID detection thinks that the date look similar to an GUID and tries to resolv this to an object. Could be a bug, you can report this to support.
As a Workaround you can also use a custom activity, which read the date from the attribute and sets it back. I think you can supress the resolving using this method.
/Peter