In all the news articles (bottom of home page), and e.g. the Orders and Settings menu, instead of showing the date as a word (e.g. "Yesterday") or the actual date, just the words "diff.ago.day" or "diff.ago.hour" shows up. Reminds me of The Princess Bride's character "Inigo Montoya".

Hello, my name is Diff.ago Montoya. Prepare to Die!

There has been multiple issues since the sale started... and GOG is... hiding in the bunkers somewhere why the place goes up in smoke?

perhaps this is a time displacement brought on by the big sale as we see all these classics it causes time to stand still as our wallets open and create's a money field in the time paradox?

P.S great film too (sees money leave wallet as more games enter my hd space :)

GOG probably has their top men working on it already.

It's just the GoGbears dismantling the spacetime continuum.

It think this is explained by the fact that Barry Allen has been messing with the timeline, again, but I'm told he will have it cleared up by the end of the season.

I also have "diff.ago.day" for my last three orders. I would be happy if GOG could solve this. (And I think Marty McFly is to blame for this, not Barry Allen.)

Has anyone noticed new receipts in Orders that say: "We've made changes to some of the items in your Library. Don't worry - no games were harmed during this process. If you're interested in further details please visit the game's forum."

I looked in the game forum but I didn't see any messages about these new receipts. What do they mean?

In general, they call it "internal installer changes".

They are injecting/will inject Galaxy wrappers in the installers.
They ruin Windows XP compatibility.
https://www.gog.com/forum/general/release_fallout_new_vegas_ultimate_edition_1c381/post153
https://www.gog.com/forum/general/release_fallout_new_vegas_ultimate_edition_1c381/post160
https://www.gog.com/forum/general/windows_xp_support_dropped_in_existing_game_cards_expeditions_conquistador/post1

As more dependencies will come in the future, there could be Wine problems for non-Windows users in the horizon.

EDIT: As for diff.ago.something, somehow they have messed with the localization strings (they use Symfony PHP framework).
If no value has been assigned to a localization variable the variable's name is returned.