Appointment Activity is showing different "start date" and "end date" for when selected "All day event" and "Duration=1 day"

Hello experts,

I do have a problem with Appointment activity. When I selected "start date" to 10/09/2017 and selected "All day event" marked ,my "end date" is setting to 10/10/2017 for "Duration =1 Day".But if I turn on the legacy form ,then its showing "start date: 10/09/2017" and "end date :10/09/2017" which counts for 1 day exactly. But if I turn off legacy form the "start date" and "end date" are showing different dates as mentioned in first case which counts for 2 days duration. Any idea why is this  showing different dates for "Duration =1 Day" and how to rectify this issue?

Any idea would be appreciated. Thanks