diff --git a/17_rewrite_git_history.md b/17_rewrite_git_history.md
index f67437f..f07850c 100644
--- a/17_rewrite_git_history.md
+++ b/17_rewrite_git_history.md
@@ -8,4 +8,6 @@ $ export GIT_COMMITER_DATE="Sun Jun 15 14:00 2014 +0100"
 $ export GIT_AUTHOR_DATE="Sun Jun 15 14:00 2014 +0100"
 $ git commit -m "so bad"
 $ git push
-```
\ No newline at end of file
+```
+
+> `GIT_COMMITER_DATE` and `GIT_AUTHOR_DATE` are environment variables
\ No newline at end of file