code: function toDelete(datetimes, options) {
  
    // We can't just a function like _.difference, because moment objects can't be compared that simply
    // and the incoming values might not already be moment objects
    var seenSurvivors = {};
    toKeep(datetimes, options).map(function (dt) {
      seenSurvivors[dt.toISOString()] = true;
    })
  
    datetimes = datetimes.map(function (dt) { return moment.utc(dt) });
  
    // The dates to delete are the ones not returned by toKeep
    return _.filter(datetimes, function (dt) { return !seenSurvivors[ dt.toISOString() ] });
  
  }
docs: Return a set of datetimes that should be deleted, out of 'datetimes`