A Property whose value is a datetime object.
Note: Unlike Django, auto_now_add can be overridden by setting the
value before writing the entity. And unlike classic db, auto_now
does not supply a default value. Also unlike classic db, when the
entity is written, the property values are updated to match what
was written. Finally, beware that this also updates the value in
the in-process cache, *and* that auto_now_add may interact weirdly
with transaction retries (a retry of a property with auto_now_add
set will reuse the value that was set on the first try).