it's currently not possible to set the DAY END before DAY START. The times HAVE to reference the same calendar date so right now, for example, you can't set DAY END to be at 1 or 2 am. The latest you can set it at is 11pm.
This is currently not just a limitation of the H2OPal app but it is also a limitation of the apps we integrate with (Apple Health, Fitbit). Unfortunately, there is no quick solution to this issue.
However, this is a feature we would still like to improve and we do have plans to support DAY END times past midnight in the future.