Yep, that's my conclusion too: storing timestamps for events in the past and up to present time - unix timestamp with optional tzid, and re-calculate local time

Yep, that's my conclusion too: storing timestamps for events in the past and up to present time - unix timestamp with optional tzid, and re-calculate local time formatting from scratch for display every time. Events in the future, do the opposite: store as yyyymmddhhiiss + tzid, and re-calculate utc from scratch for calculations every time. As a rule of thumb, at least.