Gig summary does not properly clarify start/finish dates

When a gig has a time and a finish time on a different date, the finish date isn't clarified (with arrive time left blank). The gig summary should clarify this.

To reproduce, you can use:
- Type: Gig
- Admins only: True
- Date: 2021-08-13
- Time: 19:30
- Arrive time: null
- Finish time: 2020-07-18 22:00

The finish time appears as a time but does not show a date.

This is obviously invalid data, but was the data I saw highlighted the issue. When #14 is taken on, this case should be completely prevented in gig admin, but also the summary should still be capable of showing the date to the user.