https://github.com/kamontat/ronn/pull/3
originally based on https://github.com/rtomayko/ronn/pull/98

commit 206846900163c8a538437444dfdd67a7a81e3801
Author: Bernhard M. Wiedemann <bwiedemann@suse.de>
Date:   Mon Dec 9 14:36:21 2019 +0100

    Allow to override build date with SOURCE_DATE_EPOCH
    
    in order to make builds reproducible.
    See https://reproducible-builds.org/ for why this is good
    and https://reproducible-builds.org/specs/source-date-epoch/
    for the definition of this variable.
    
    It prioritizes the variable over mtime
    because openSUSE's `tallow` package touched a .md file with a patch.

diff --git a/lib/ronn/document.rb b/lib/ronn/document.rb
index d42e2f9..6590c92 100644
--- a/lib/ronn/document.rb
+++ b/lib/ronn/document.rb
@@ -171,6 +171,7 @@ module Ronn
     # the current time.
     def date
       return @date if @date
+      return Time.at(ENV['SOURCE_DATE_EPOCH'].to_i).gmtime if not ENV['SOURCE_DATE_EPOCH'].nil?
       return File.mtime(path) if File.exist?(path)
       Time.now
     end