Auto merge of #1292 - RalfJung:time, r=RalfJung

sanity check time values a bit more

tests/run-pass/time.rs

@@ -3,16 +3,22 @@
 use std::time::{SystemTime, Instant};
 
 fn main() {
+    // Check `SystemTime`.
     let now1 = SystemTime::now();
     // Do some work to make time pass.
     for _ in 0..10 { drop(vec![42]); }
     let now2 = SystemTime::now();
     assert!(now2 > now1);
     let diff = now2.duration_since(now1).unwrap();
-    assert!(diff.as_micros() > 0);
     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);
 
+    // Check `Instant`.
     #[cfg(not(windows))] // `Instant` shims not yet implemented on Windows
     {
         let now1 = Instant::now();
@@ -24,9 +30,11 @@ fn main() {
         #[cfg(target_os = "linux")] // TODO: macOS does not support Instant subtraction
         {
             let diff = now2.duration_since(now1);
-            assert!(diff.as_micros() > 0);
             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);
         }
     }
 }