As I said before, I had realized that date_default_timezone_set() was the culprit.
I removed every occurrence of it in my themes/plugins and made a function instead which takes a timestamp or a Ymd as an argument and returns several different date/time formats (also passed via an argument) taking into account the timezone offset from WP. Now I’m using my function instead of date() or strftime() in all my WP projects. Everything seems to run smoothly, including ACF Pro.
