PetarB → created an issue.
I can confirm issue was resolved on PHP 8.0 with patch #13 thanks dariogcode
@TR I love your work and have so much respect for you but let's not pretend that Drupal 7 is going anywhere soon.
Just look at this:
https://www.drupal.org/project/usage/drupal →
As to the OP and other people looking at this thread, its an issue I've come up against a lot. We've always converted dates to U (epoch time), done our date math, (ie, add durations etc), convert up and back to native datetime, contrary to the OP saying this isn't a solution. Actually, it's the only real guaranteed solution. You have to take responsiblity here because date are HARD... they don't follow periodicity or predictability. Don't rely on Rules for date math, there's so many issues that can come up, including plenty of issues that are completely outside the scope of Rules.