calendar.format: rfc3339, don't drop seconds decimals

This PR is not meant to be merged, it show a problem with rfc3339. Right now, it drops the second decimals of a date. Here are the problems with this patch:
   - I USEd a vocabulary (decimals) from extra in one from basis (calendar.format).
   - I didn't use >float to have arbitrary precision, not sure if that's a good idea or not