I recently obtained the Coaster Addict achievement - Steam lists my unlock time as Jul 10 @ 5:42am (this is in the Eastern US time zone). However, in the game itself, my collection book lists the time as 1/1/1970 at 12:00 AM (UNIX 0, or maybe UNIX 1 through 59).
I’m not actually sure why this happened. I briefly walked away from my computer at the time that the achievement unlocked, and came back in AFK mode - I’m not sure if it may have unlocked right before going AFK and this had something to do with it?