Dates are kind of funny, in that they are stored as integers but interpreted as dates when being accessed. To further muddy the waters, Excel doesn't always do a great job of intuiting the value of a date field (or any field, for that matter).
Depending on the underlying database, the "0" date can be all sorts of things. Can you clarify what DB platform the SDE is running on?
Also, can you try exporting to a text-based format and looking at the raw values before Excel tries to interpret them?
- Josh Carlson
Kendall County GIS