filter-branch --env-filter GIT_AUTHOR_DATE

To fix invalid timezone info in a repo I ran
git filter-branch --env-filter '
  GIT_AUTHOR_DATE=`echo ${GIT_AUTHOR_DATE}|sed s/+0000/-0800/`' HEAD

This fixed the invalid entries but the new timezone is -0700
instead of -0800. Is this expected?

git version 1.7.4.rc2