I've run into this at work and I handle by always add timedeltas in UTC or just design everything to work in UTC.

Unfortunately, some people dislike the former because it's 3 lines of code (convert to UTC, add delta, convert to local). I've had the same data scientist reject 3 MRs fixing this sort of issue in his code. At least it only causes a once per year error or a blip that no one notices.

Always using UTC doesn't work always depending on requirements. The worst was interacting with an API that required precisely every hour of the following day to be sent without good documentation.