UNION With A Max On Split Date/Time Fields

I have 2 tables, each with one ID field, a separate
Date and Time fields and a number of other fields.
The tables contain duplicates on the ID field.
I want to do a UNION keeping only the record with the latest
Date and Time.

This would work:
SELECT MyTab.myKeyField, Max(MyTab.myDate) AS myDate
FROM (SELECT myKeyField, myDate
from Table1
union
SELECT myKeyField, myDate
from Table2) AS MyTab
GROUP BY MyTab.myKeyField

But is only taking care of Date, not Time (some records have
the same date but different times)
The other problem is, when I add more fields, I have to
include them in the GROUP BY clause, and this way I end up
with duplicates (because some other fields have different
values)

Is there a way to do this?