For a single repo:
git log --all --after="<date> 00:00" --before="<date> 23:59" --author="<author>"
<date>
can be in any common format. <author>
is case sensitive, and also checks the email address.
Note:
This looks at the committer date (when was this commit applied to this repository/branch), not the author date (when was this content first committed). This is important if you have rebased and/or cherry-picked. There does not appear to be equivalent functionality for author date.
Edit:
To get this log for a set of repositories, you can run:
for dir in repo1 repo2 ... repo_n; do
git --no-pager --git-dir="$dir/.git" log --all \
--after="<date> 00:00" \
--before="<date> 23:59" \
--author="<author>"
&& echo # to add a blank line between logs
done