People digging around in the code of Metallica XX noticed ties to promoter Danny Wimmer Presents, who puts on festivals like Louder Than Life, Sonic Temple Art & Music Festival, Aftershock. Now The PRP points out all those websites have been "hacked" and will have an announcement posted on – ready? – Thursday, October 10 at noon.
My initial guess for the clock's countdown result was the announcement of S&M² coming out on CD/DVD. The website is called Metallica XX, S&M was released in 1999 and S&M² is presumably coming out in 2019. So there's my reasoning for the 20. It also helps that S&M² is hitting theaters on October 9 and an announcement like that the day after makes sense. Plus, and I hate to bring this up, but James Hetfield is in rehab and who knows how long his recovery is going to take. Now there's a very reasonable alternate guess as to what the clock is for.
Stay tuned, but it looks like Metallica is planning for 2020 to be a pretty big year (hopefully).