<label for="appt">Choose a time for your meeting:</label>

<input type="time" id="appt" name="appt" min="09:00" max="18:00" required />

<small>Office hours are 9am to 6pm</small>

label {
  display: block;
  font:
    1rem 'Fira Sans',
    sans-serif;
}

input,
label {
  margin: 0.4rem 0;
}