mapbox/togeojson — pull request #34 by Swatinem (2014-02-20 11:27:58+00:00)
Title: add time from gpx as 4th parameter

Not sure what your policy is no this. GeoJSON allows any additional number of elements, so having the time there makes sense.
But I do have some request for feedback: When we have time, but no elevation, should I use 0 or null for elevation?
The geojson spec is not that specific about it, it just says "A position is represented by an array of numbers."