Shortcut for calling Core_extended.Unix.strftime without having to create a Time.t and convert it to a Unix.tm.
format "%Y-%m-%d" t will return "YYYY-MM-DD"
format "%Y-%m-%d" t
Convert a time span to a human-readable string, e.g. "1:23:45.778" (versus "1.396h" from Time.Span.to_string).
Time.Span.to_string