Sleep goals are a bit awkward because the sleep typically does happen across a day boundary. If you're not creeping along on the exact edge of the road, then it doesn't much matter how we decide to slice the day and where we count the data, but it does lead to weird stuff like this. If we used the end date for data entry it would almost always wind up with one sleep per calendar date, except in the case of napping.
I think I decided to use the date at the start reasoning that it was slightly more generous in the case where the user is riding close to the edge of the road. Though, only marginally, because if you go to bed before midnight with the goal in the red, then the goal will derail before the data is entered on the goal. Though once it is entered it might retroactively save you, though you'd still have to email support...
This is sounding rather convoluted as I try to describe it, eh?
I was also going to claim that "most sleep tracking apps" refer to your sleep as happening yesterday, e.g. I'm currently using my pebble for sleep tracking, so I opened up the app expecting to see it tell me that I slept ~7 hours "yesterday", but I was wrong. They are refering to my sleep "today". So maybe I'm not following any conventions there?