A due date tells Memento when to bring something back. Most things need a day, not a time.
Set a due date from the item's detail view, from the long-press menu, or from the bar above the keyboard while you're capturing.
Dates are day-only by default. Turn on Time when something has to happen at a specific moment — a call, a meeting, a medication.
Plenty of what you capture never needs a date. A due date is for things that have to come back at a particular moment — everything else can simply wait in a list until you want it.
Note: Items with a due date leave your Inbox and show up in Today or Upcoming instead.