They are not. See: https://git-scm.com/docs/git-format-patch#_discussion

To quote the relevant bit:

"The patch produced by git format-patch is in UNIX mailbox format, with a fixed "magic" time stamp to indicate that the file is output from format-patch rather than a real mailbox".

The date "Mon Sep 17 00:00:00 2001" is that magic date.