diff --git a/src/lib.rs b/src/lib.rs
index 5354e68..65d09f2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -21,7 +21,7 @@ mod convert {
     /// million years.
     pub fn millis(duration: Duration) -> u64 {
         // Round up.
-        let millis = (duration.subsec_nanos() + NANOS_PER_MILLI - 1) / NANOS_PER_MILLI;
+        let millis = duration.subsec_nanos().div_ceil(NANOS_PER_MILLI);
         duration
             .as_secs()
             .saturating_mul(MILLIS_PER_SEC)