Since 9e21dd9, Gem::Package::TarWriter#add_file adds the file to the tar with Gem.source_date_epoch for its mtime. This behavior breaks the code depending on the previous add_file behavior. Therefore, add_file accepts mtime as an argument, and uses Gem.source_date_epoch if not specified. https://github.com/rubygems/rubygems/commit/7020ea98a0