Gem::Commands::RebuildCommand#get_timestamp

# File tmp/rubies/ruby-4.0.0/lib/rubygems/commands/rebuild_command.rb, line 120
def get_timestamp(file)
  mtime = nil
  File.open(file, Gem.binary_mode) do |f|
    Gem::Package::TarReader.new(f) do |tar|
      mtime = tar.seek("metadata.gz") {|tf| tf.header.mtime }
    end
  end

  mtime
end