Yes. Just count seconds and have a handy database of timezones and another of leap seconds. Yes, your computations of future times will be wrong if you can't predict leap seconds accurately (indeed, you cannot!). But if we're talking about a calendar then you want to store wall-clock times and every so often recompute a near-in-the-future time at which to resolve (into TAI) all relevant near-in-the-future times in the calendar so you can fire off reminders/alerts/whatever at the right times.