How to? Add date/time to custom tpipe based BTM

I would like to add the last run date/time to filter out files that have already been processed - on the line from the attached BTM:

for %fn in (*.eml *.lst *.txt) (

but not sure how best to do it. I would aloso think it would be a user registry variable - like

SET EMailAddrsRunTime=%_DATETIME

then the contents is used to filter out files earlier then that date/time....