Function occurrence_at 

fn occurrence_at(
    text: &str,
    start: i64,
    zone: &TimeZone,
) -> Option<(usize, bool)>

Which occurrence of a file’s event starts at start (Unix seconds), as the agenda shows it: the place of the VEVENT holding it among the text’s VEVENTs (calcard keeps them in the text’s order), and whether that one is a changed occurrence (RECURRENCE-ID). Times without a zone are in zone.