Function format_time_of_day 

pub fn format_time_of_day(nanos_from_midnight: i64) -> String

Format a time-of-day (nanoseconds from midnight) as HH:MM:SS, with a trailing sub-second fraction only when nonzero (trailing zeros trimmed) — e.g. 07:30:45, 16:00:00, 07:30:45.5. The clock face is lossless to the nanosecond; the input is normalised modulo a day so a negative or overflowing value still maps onto [00:00:00, 24:00:00).