class ActiveSupport::FileUpdateChecker

reloading is not triggered.
healthy to consider this edge case because with mtimes in the future
can happen for example if the user changes the clock by hand. It is
Files with a mtime in the future are ignored. Such abnormal situation

if the array is empty.
This method returns the maximum mtime of the files in +paths+, or +nil+

def max_mtime(paths)
  time_now = Time.now
  paths.map {|path| File.mtime(path)}.reject {|mtime| time_now < mtime}.max
end