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

<input type="time" id="appt" name="appt"
       min="9:00" max="18:00" required>

<span class="note">Office hours are 9am to 6pm</span>

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

input,
label {
    margin: .4rem 0;
}

.note {
    font-size: .8em;
}