Legacy's saying not to ignore the zero -- what if the zero is a clue about the clue?
It's serving as a left/right divider in the country flags, and something else that's a zero that divides left/right, or east/west, is the prime meridian.... I looked up the hemispheres for each country, and five are (at least mostly) in the western hemisphere: Brazil, Ghana, Haiti, Honduras and Iceland. Conveniently these countries also all start with letters that translate to single digits using the 26 cipher, which produces the number 27889. If you plug that into excel and reformat as a date you get 5/9/1976 -- I couldn't find much for episode air dates with that, but if you switch the month and day (and SkelEthan on the discord did point out that Legacy has liked a tweet about the US being the only country to use the MMDDYYYY format),
9/5/1976 was the air date for
an episode of The Muppet Show, and Legacy has tweeted about the Muppets before so it's not a huge reach to think that could be it.
The celebrity host of that episode was Joel Grey, and the main character of TLoU is named Joel...