Interesting, thank you very much for the detailed explanation.
I hadn't noticed it was only a warning, so I can definitely ignore that then.

I'll mark this as solved.

Thanks again!

I set up a debian/apt feed and I can't seem to be able to use it.

I ran the following commands on the client machine:

wget -O "deb.gpg" https://<URL>/debian-feeds/buster.pub && sudo apt-key add "deb.gpg"
echo "deb https://<URL>/ buster main" | sudo tee /etc/apt/sources.list.d/proget.list

And then running sudo apt update returns the following:

W: Invalid 'Date' entry in Release file /var/lib/apt/lists/partial/<URL>_dists_buster_Release

I managed to output the file and it looks like this:

Components: main
Architectures: amd64
Codename: buster
Description: Debian Buster
MD5Sum:
 main/binary-amd64/Packages 2936 ac4e38f0cb4bf881ae144c869cbe6a31
SHA1:
 main/binary-amd64/Packages 2936 e5f5f4aa00f4c30ece4d3e1d8fe6376ab7732c52
SHA256:
 main/binary-amd64/Packages 2936 89c24f7996767b72b4244371e2d570993926f2628bb6dca6ff9f2ec197ab7c1c
SHA512:
 main/binary-amd64/Packages 2936 611e7e1155511ab081ffe8bdc5b39a1a776d7647ff212ed23349c7abd28d05dad038be75bdb53a46871dec01215e795be7919da8ebd1de03ff6c271a1410d241

Any clue as to what's happening?

Thanks