Now that RFC 7946 has been published, I'm returning to my work on describing event-like GeoJSON features at https://github.com/sgillies/geojson-events. My goal is to come up with a representation for instants and intervals of time that does for "when" what GeoJSON has already done for "where." Applications like the USGS earthquake feeds are what I have in mind for this GeoJSON extension; geojson-events is not suited for GPS tracks or telemetry of moving objects, these kind of measurements require a different model.
I would be grateful for your comments on geojson-events. Please check out the project repository and let me know if this GeoJSON extension is or isn't useful to you.