In addition to --date=(relative|local|default|iso|iso-strict|rfc|short|raw), as others have mentioned, you can also use a custom log date format with

--date=format:'%Y-%m-%d %H:%M:%S' # committer's timezone
--date=format-local:'%Y-%m-%d %H:%M:%S' # current user's timezone
This outputs something like 2016-01-13 11:32:13.

NOTE: If you take a look at the commit linked to below, I believe you'll need at least Git v2.6.0-rc0 for this to work.

In a full command it would be something like:

git config --global alias.lg "log --graph --decorate -30 --all --topo-order --date=format-local:'%Y-%m-%d %H:%M:%S' --pretty=format:'%C(cyan)%h%Creset %C(black bold)%ad%Creset%C(auto)%d %s'"
I haven't been able to find this in documentation anywhere (if someone knows where to find it, please comment) so I originally found the placeholders by trial and error.

In my search for documentation on this I found a commit to Git itself that indicates the format is fed directly to strftime. Looking up strftime (here or here) the placeholders I found match the placeholders listed.

The placeholders include:

%a Abbreviated weekday name
%A Full weekday name
%b Abbreviated month name
%B