Yeah... I pretty early in my career firmly cemented on a couple things with date-times. It's either a date-time + zone/location detail or always seconds from unix epoc or UTC in iso-8601 style (later JSON's adopted default) across the wire. Since most systems and JS convert pretty easily between UTC and local.

Same for storage details. I started using the 8601 style mostly in file/log naming so they always sorted correctly, this kind of carried over into my code use pre-dating JSON spec.

Doing the above saves a lot of headaches... I'd also by convention use a few utility scripts for common formatting and date changes (I use date-fns now mostly), that would always start with dtm = new Date(dtm); before manipulation, returning the cloned dtm.