On the edit visit page, the startDatetime and stopDatetime fields have date pickers, with no time component. This is bad because the most common use case is for OpenMRS to automatically create day-long visits that start at midnight and end at the last second of the day. Currently, if you edit one of those visits, the time is stripped off of endDatetime.
We probably need to introduce a new timepicker widget into core for this. (In HTML Form Entry I did something relatively ugly with dropdowns 0-23 for hour and 0-59 for minute. I looked around and found some jquery plugins that embellish the jquery-ui datepicker, but didn't like any of the time. Whoever picks up this ticket should re-review them though.)