AFAIK it's not the compression date but instead the last modified date stored in the filesystem metadata. It's likely there such that `gzip file; gunzip file.gz` keeps the mtime information intact.

The problem's it's any date so which date is stored depends on the library and the datasource. If you're gzipping and caching dynamic content there's no filesystem mtime, so you'll probably get the compression time instead (unless the remote end or library defaults to 0)