Date field affected by Merge

[StephenGonzales]

I am running 10.0, SP5 and attempting to merge 12 point shapefiles. Each file contains AIS data for one month of the year. The merge completes successfully but the dates in my date field become corrupt. The date for most records will become modified and either be blank, change to "12:00 am" or become some other date, e.g. 12/21/7200. When I merge 2 or 3 together no change occurs. But, if I add a fourth or more, the dates modify. Any idea why?
The number of points in each file ranges from approx. 886,000 to approx. 1,290,000.

Thanks.

[LeeHightower]

do the date fields of different tables have the same format?

[StephenGonzales]

Yes. And the merge will work without date modification for any combination of 3 months. But when I try to merge more than 3, the errors occur.

[DarrenWiens2 · Accepted solution]

Have you tried saving the output as a geodatabase feature class? Feature classes and shapefiles handle dates differently.

[StephenGonzales]

That appears to have done the trick. The merge was faster, too.
Thanks for the help.