I ran across this while running some sanity checks on time. Quickcheck immediately found a bug, and as I'd modified the code from std I knew there was a bug here as well.

tl;dr this code fails (playground)

use std::time::Duration;

fn main() {
    assert_eq!(
        Duration::new(1, 1).checked_div(7),
        Some(Duration::new(0, 142_857_143)),
    );
}

The existing code determines that 1/7 = 0 (seconds), 1/7 = 0 (nanoseconds), 1 billion / 7 = 142,857,142 (extra nanoseconds). The billion comes from multiplying the remainder of the seconds (1) by the number of nanoseconds in a second. However, this wrongly ignores any remaining nanoseconds. This PR takes that into consideration, adds a test, and also changes the roundabout way of calculating the remainder into directly computing it.

Note: This is not a rounding error. This result divides evenly.

@rustbot label +A-time +C-bug +S-waiting-on-reviewer +T-libs