gpsbabel.exe -p "" -t -i garmin -f com3: -x track,start=20070215000000,stop=20070215235959,fix=3d,course,speed -o nmea -F -

trackfilter-init: Found track point without time

Only active tracks in Etrex have time.  Saved tracks are compressed by the unit and have the timestamp and altitudes thrown out.  Therefore, options like the one you've used that require time are doomed to fail.

Olaf, could we loosen the restriction for this case and just assume that these mangled trackpoints have a time of zero (they do, internally) and would therefore fall outside the range of the filter?