Custom metadata field with date filtering like built-in Date field

Question

I've added a custom metadata field OK and now want to add an additional custom metadata field that contains the date the photos were exported by my plugin.  The types of metadata values don't include a date field.
I've tried using a string with the value returned by os.date() and that does indeed add the date and time but what I'm really looking for is a way to add a date field so it will have the same behaviour that the built-in Date field does. 
When I use Library Filter: Metadata, the Date column shows the date with a series of disclosure buttons in a hierarchy, e.g.,
Date
All (1 Date)
V 2009
   V November
      V 27-Friday
Is this some magic for the built-in field or is there some way of formatting the date so Lightroom knows it's a date and can treat it the same way as the built-in date?
Abe

Answer 1

Thanks.  I've just added a date string.  It's not an ideal solution but it's the best I can manage at this time.  Maybe in LR3.
Abe