foo.js

var times = {
  // Last day available to register
  end: {
    'James': r.time(2016, 7, 3, '-04:00'),
    'John': r.time(2016, 7, 2, '-04:00')
  },
  // First day available to register
  start: {
    'James': r.time(2016, 7, 2, '-04:00'),
    'John': r.time(2016, 7, 1, '-04:00')
  }
}

// Doesn't work
r
  .db('camp')
  .table('campers')
  .getAll(r.args(['James', 'John']), {index: 'first_name'})
  .filter( (doc) => {
    return r
      .expr(times.start)(doc('first_name'))
      .ge(doc('time'))
      .and(r
        .expr(times.end)(doc('first_name'))
        .le(doc('time')
      ))
  })
  
// Doesn't work
r
  .db('camp')
  .table('campers')
  .getAll(r.args(['James', 'John']), {index: 'first_name'})
  .filter( (doc) => {
    return doc('time')
      .during(
        r.expr(times.start)(doc('first_name')),
        r.expr(times.end)(doc('first_name'))
      )
  })