message: tighten Instance sanity check
diff: // compile-flags: -Zmiri-disable-isolation
  -use std::time::{SystemTime, Instant};
  +use std::time::{SystemTime, Instant, Duration};
  +
  +fn duration_sanity(diff: Duration) {
  + // On my laptop, I observed times around 15-40ms. Add 10x lee-way both ways.
  + assert!(diff.as_millis() > 1);
  + assert!(diff.as_millis() < 500);
  +}
  fn main() {
  // Check `SystemTime`.
  let now1 = SystemTime::now();
  + let seconds_since_epoch = now1.duration_since(SystemTime::UNIX_EPOCH).unwrap().as_secs();
  + let years_since_epoch = seconds_since_epoch / 3600 / 24 / 365;
  + let year = 1970 + years_since_epoch;
  + assert!(2020 <= year && year < 2100);
  // Do some work to make time pass.
  for _ in 0..10 { drop(vec![42]); }
  let now2 = SystemTime::now();
  assert!(now2 > now1);
  + // Sanity-check the difference we got.
  let diff = now2.duration_since(now1).unwrap();
  assert_eq!(now1 + diff, now2);
  assert_eq!(now2 - diff, now1);
  - // Sanity-check the time we got.
  - let seconds_since_epoch = now1.duration_since(SystemTime::UNIX_EPOCH).unwrap().as_secs();
  - let years_since_epoch = seconds_since_epoch / 3600 / 24 / 365;
  - let year = 1970 + years_since_epoch;
  - assert!(2020 <= year && year < 2100);
  + duration_sanity(diff);
  // Check `Instant`.
  let now1 = Instant::now();
  @@ -24,10 +31,9 @@ fn main() {
  for _ in 0..10 { drop(vec![42]); }
  let now2 = Instant::now();
  assert!(now2 > now1);
  + // Sanity-check the difference we got.
  let diff = now2.duration_since(now1);
  assert_eq!(now1 + diff, now2);
  assert_eq!(now2 - diff, now1);
  - // Sanity-check the difference we got.
  - assert!(diff.as_micros() > 1);
  - assert!(diff.as_micros() < 1_000_000);
  + duration_sanity(diff);
  }
  