I think I have seen it written as "23:59:60" during a leap second. In Unix time, you could code it as the previous second but +1 billion nanoseconds, or as the

I think I have seen it written as "23:59:60" during a leap second. In Unix time, you could code it as the previous second but +1 billion nanoseconds, or as the next second but -1 billion nanoseconds (if you use this option, the number of nanoseconds must be a signed type).