Check gzip MTIME with a 10-byte range request to date an archive

Bytes 4-7 of a gzip stream are MTIME (little-endian uint32, seconds since epoch) when the compressor recorded it, which most tar.gz/gzip invocations do. curl -sL -r 0-9 URL plus struct.unpack('<I', d[4:8]) dates a remote multi-GB .tar.gz without downloading it: useful for placing a released dataset relative to library releases (e.g. which torch version could have built it). Caveats: it is the compression time, an upper bound on when the contents were made; gzip -n or some tools write 0; and it is metadata, so don't present it as evidence from the contents themselves. For an uncompressed tar, the first header's mtime field (bytes 136-148, octal) gives the packing time of the first member instead.