That's the second time that's happened in date-related ASs... The trick seems to be to force a recompile of the script to recreate the date object, or you can do as you have and build it every run.
If you go back to my version you'd change the line
set theDate to date "Thursday 1 January 1970 at 00:00:00"
to
set theDate to date "1/1/1970"
replacing 1/1/1970 with the representation of 1 January 1970 in your locale's format. Hit the Enter key and the script will recompile to give you the "full" date object.
Is it working now you've fixed the error? The KM icon in the menu bar should spin for as long as it takes for the macro to complete and show the "Display Text" with results.