Profile displaying GPS time/date - doesn't work with Garmin Alpha 100 collar data

I love the apparently-new feature where a profile of an imported GPS track (.gpx file) shows you the date and time at any given point along the track.  However, using the Garmin Alpha 100 I find this only works for tracks recorded on the handheld; dog-collar tracks don't show the date and time in the profile, even though the data is in the imported .gpx file.