To get YYYY-MM-DD format for dates git log --date=iso is used. To force dates in your local time zonegit log --date=local is used.
YYYY-MM-DD
git log --date=iso
git log --date=local
A lot of time passed since the question, but this link was one of the first in google, and it had no answer.
So for people who find it: what you want is --date=format-local:'%F' (or %F %R for time too).
--date=format-local:'%F'
%F %R