UTC is fine for single events. For the recurring events, UTC + timezone alone is useless, you need to know the DST rules too to make correct calculations over t

UTC is fine for single events. For the recurring events, UTC + timezone alone is useless, you need to know the DST rules too to make correct calculations over the DST changes. vCal has spec for DST rules, but you don't want to store them for every event. You store a location or "DST zone" of your event and have DST rules in separate database (they need to be updated as DST rules can change) When I worked on calendar applications, there was not commonly agreed way to transfer DST zones between systems, but single DST rules could be transferred as part of vCal entry. Microsoft apparently implemented their own integer code for every "DST zone" and used it to transfer events correctly between Microsoft systems (e.g. sending meeting invitations by email from Outlook to Outlook). Things might have changed since I worked on this area, I haven't checked the current status.