Try the following:

  add an interval property to the 
issue

  create the entries in the issue.search and 
issue.index
     to allow the interval property to be 
displayed.

  enter some issues with values for the 
interval.

  create a search displaying the interval 
property.

  in the issue.index display, reverse the sort on 
the
      interval property.

When I do this, I see no change in 
the ordering despite
having mixed +2d, +7d and empty interval 
values.

The "sort" order is currently +2d, 4 empty values, 
+2d,
empty value, +7d, 2 empty values. The order stays 
the
same if I click on the decending order checkbox 
and
redisplay the page. I used:

      
leadtime=Interval()

to create the interval 
property.

Is this just me or is something dicey?