Scored 17/28. Thank you, this is absolutely cursed! It's probably a good time to go and check out the Temporal stuff (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...).
Implementations are ongoing and open source, so you can contribute if you want it to come faster! V8 is currently aiming to use https://github.com/boa-dev/temporal IIRC.
I've had to develop interfaces built entirely around date and time operations, and it was painful not so much because of the awful and broken date parsing in JS (although it definitely doesn't help), but rather because of the inherent complexity of how we represent calendar dates and times, and how we build abstractions around these representations. Definitely NOT looking forward to developing that booking system at work...