CalendarInterval

When the checkout method is takeout and latch, a user can choose a time between start and end (including the boundaries).

But if the checkout method is delivery and use_intervals is enabled, a user cannot choose a time. He/she needs to choose an Interval object (with fixed start and end times) provided in `Day`. The start time of the selected interval should be saved in delivery.time, and the end in delivery.end_time. 
When use_intervals is disabled, a user can choose a time between start and end (including the boundaries), and set end_time to null. You should display ± 15 minutes.

You can determine if use_intervals is enabled by examining the response of the `POST /checkout` endpoint

Examples

Example 1

{
    "start": "12:00",
    "end": "12:00"
}