Not 100% sure this has been included or not in the most recent release of Meteor, but for a time there was a problem when trying to store dates in the Meteor Session object due to JSON-compatibility issues and the hot code push. For those whos still running on a version experiencing problems storing dates in session, the way i worked around this was simply to use moment with two helper methods.

getChosenDate = function() {
  return moment(Session.get('chosenDate'));
};

setChosenDate = function(date) {
  Session.set('chosenDate', date.valueOf());
};