When you speak of "form", I assume that you are talking about page properties or component properties, is that correct?
In this case, the Sling Post Servlet handles the conversion of the date into a property with the JCR type "date". And the way, which CRXDE Lite is displaying it (I assume that you took the formatting from there) is one way to display it. That means, your simplified approach to store "time" is handled by the system in a way to store it as full "date object" including minutes, seconds, timezone etc.
Tat way you should also treat the object as full "date object" and format it according to your needs on the client side.
As Jorg suggested you cans ave date as it is but manipulate during display.
Or you can you below document to set date formats while saving and displaying? Please check if it helps.