Why are the md5 checksums for download archives different?

Hello everybody,

We have several build servers that bake the firmware. Recently I set up another build server and was surprised that the files in the download folder have a different md5 checksum than in the building specification.
So the tarballs are fetched directly from gitlab-server via the link in the “download button”.
another build servers share the download cache. I did not notice that the check sums have changed.

I have checked the contents, they are identical. With the help of https://ide.kaitai.io I have found out that the differences are in fileModTime=0x081e and fileModeDate=0x1451.

So how are archives built? and which date is stored in the fileModTime and fileModDate fields?
Date of the commit or of tag creation or …?

Regards,
Andreas