JSON itself does not specify how dates should be represented, but JavaScript does.
You should use the format emitted by Date's toJSON method:
2012-04-23T18:25:43.511Z
H. Fliegel, and R. DiEsposti. Proceedings of the 28th Annual Precise Time and Time Interval Systems and Applications Meeting, page 189--200. Reston, Virginia, (December 1996)