[Geojson] How to define a timestamp?

[Matthias Baldauf]

Dear GeoJSON community,

I'd like to use GeoJSON for implementing a tracking service. Besides the 
user's position, I also need to specify a timestamp describing the time 
of the last update.

I couldn't find a way to define a timestamp in a GeoJSON object so far. 
Can anyone tell me the favorable way to do so? Are there any plans to 
include a timestamp in GeoJSON, e.g. for specifying the up-to-dateness 
of location data?

Regards,
Matthias