I Forgot to Hit End Until I Drove Away
That's ok! Slopes added a trim feature because this happens to all of us.
When you're done recording you'll see a "Trim Recording" button on the save screen. If you already saved, you can edit your day by hitting the "..." in the top right when you're looking at the day. From there you'll be able to pick your real start and end time.
Note: when you trim and then save, any data you cut out will be removed for good. Trim responsibly.