Any way to amend AuthorDate?

Is there any way to amend a commit and change only the Author Date?

For instance, I have a binary file that's been checked in in two
consecutive commits. After squashing them into one, the new commit would
keep the timestamp of the earlier one of the two original commits. I
would like it to have either the later timestamp or whenever the squash
happens.

Thanks for any pointers.
-- 
Jing Xue