DateRange editing: inherit the 'mode' attribute.
Document it.
Formatting.
Add units to Object panels.
Allow the mode attribute to specify... a mode.

It's confusing, but now you can say mode="mode=time" to get the time editor.
This is to support using the DateRange object with mode=date, if desired.

Although, I just realized this will screw with the 'about now'
calculation it does.
Sheeeit. Maybe the editor itself should be handling the conversion from 0
that we frequently do when editing dates...
Added some notes.

Possible future enhancements. I'm not going into the rabbit hole now.
Rethink: do the conversion from 0 in the editor.

Now annotating a raw long might make sense because you don't
need to do the 0 conversion yourself. Also it will correctly
set the initial value based on the mode of the editor, so if in
date-only mode the filled-in values will be at midnight.
Document our 0 conversion.

Also write 1 instead of 0, which should be fine. I doubt we really
need any hard cutoffs at the epoch.