How to (re-)create .git/logs/refs

Hi !

I have an old SVN project with lots of commits from 2005 to 2008 converted. This was a month ago. In the mean-time, I've made some commits from git as well.

"git log" shows both types of commit quite nicely.

"git checkout <sha1>" works also correctly, for arbitrary ancient SHA1s.

"git checkout @{2007-04-01}" doesn't work. I get an error like this:

  warning: Log for '' only goes back to Thu, 8 May 2008 09:35:38 +0000.
  fatal: bad object @{2007-04-01}

However, "git checkout @{2008-05-01}" works, because this commit come from git direct, not implicitly from SVN. It seems that neiver "git svn clone" nor "examples/git-svnimport.perl" creates/updates .git/logs/refs when it creates commits from an old SVN repository.

So my question:

a) is there a way to re-create .git/logs/refs ?

b) is there a simple way to get a list of ISO-dates and
   associated SHA1?  Then I could use a little python code to
   determine the SHA1 id that corresponds to some date (I need
   to access old versions of the source via bitbake and/or
   www.openembedded.org, so adding a custom fetch method with
   some python magic is a non-brainer).