Using offset CSYS with expression?

I revised an existing Datum CSYS to use an "Offset CSYS" type so I could rotate it about the z-axis.  I also created an expression for the rotation angle to make it easier to change, but for some reason that is not working.  The datum will rotate using the Datum CSYS dialog, but it doesn't respond to changes in the expression.  Also, if I edit the datum it always shows the Type as "Dynamic" (not Offset).  So apparently there's something I'm not undestanding with this.  What am I missing?

Thanks for the help.