You can subtract the date from now() and compare it to a duration. Give this a try.
?s :myProperty ?ts
filter(now() - xsd:datetime(?ts) < "P6M"^^xsd:duration)
I'm not quite sure why but I had to add the xsd:datetime cast even though the dummy data I was using to test the query was already a xsd:datetime.
I had written a custom function to do almost exactly what you had written originally. It would extract the date from a natural language string so you could say somethinglike
FILTER(?ts < myFunc:speakTime("6 months ago")
I'll have to dig it up and find out where I left off on that.