def zip_entry_for_part(part)
      timestamp = Zip::DOSTime.at(@core.created.to_i)
      Zip::Entry.new("", part[:entry], "", "", 0, 0, Zip::Entry::DEFLATED, 0, timestamp)
    end
def zip_entry_for_part(part)
      timestamp = Zip::DOSTime.at(@core.created.to_i)
      Zip::Entry.new("", part[:entry], "", "", 0, 0, Zip::Entry::DEFLATED, 0, timestamp)
    end
Generate a Entry for the given package part.
 The important part here is to explicitly set the timestamp for the zip entry: Serializing axlsx packages
 with identical contents should result in identical zip files – however, the timestamp of a zip entry
 defaults to the time of serialization and therefore the zip file contents would be different every time
 the package is serialized.

 Note: {Core#created} also defaults to the current time – so to generate identical axlsx packages you have
 to set this explicitly, too (eg. with `Package.new(created_at: Time.local(2013, 1, 1))`).

 @param part A hash describing a part of this pacakge (see {#parts})
 @return [Zip::Entry]