diff --git a/src/libcore/time.rs b/src/libcore/time.rs
index 4777b4356376..cd98f51f5cdb 100644
--- a/src/libcore/time.rs
+++ b/src/libcore/time.rs
@@ -517,16 +517,16 @@ impl Mul<f64> for Duration {
 
     fn mul(self, rhs: f64) -> Duration {
         const NPS: f64 = NANOS_PER_SEC as f64;
-        if rhs.is_sign_negative() {
-            panic!("duration can not be multiplied by negative float");
-        }
         let nanos_f64 = rhs * (NPS * (self.secs as f64) + (self.nanos as f64));
         if !nanos_f64.is_finite() {
             panic!("got non-finite value when multiplying duration by float");
         }
         if nanos_f64 > MAX_NANOS_F64 {
             panic!("overflow when multiplying duration by float");
-        };
+        }
+        if nanos_f64 < 0.0 {
+            panic!("underflow when multiplying duration by float");
+        }
         let nanos_u128 = nanos_f64 as u128;
         Duration {
             secs: (nanos_u128 / (NANOS_PER_SEC as u128)) as u64,
@@ -541,16 +541,16 @@ impl Mul<Duration> for f64 {
 
     fn mul(self, rhs: Duration) -> Duration {
         const NPS: f64 = NANOS_PER_SEC as f64;
-        if self.is_sign_negative() {
-            panic!("duration can not be multiplied by negative float");
-        }
         let nanos_f64 = self * (NPS * (rhs.secs as f64) + (rhs.nanos as f64));
         if !nanos_f64.is_finite() {
             panic!("got non-finite value when multiplying float by duration");
         }
         if nanos_f64 > MAX_NANOS_F64 {
             panic!("overflow when multiplying float by duration");
-        };
+        }
+        if nanos_f64 < 0.0 {
+            panic!("underflow when multiplying float by duration");
+        }
         let nanos_u128 = nanos_f64 as u128;
         Duration {
             secs: (nanos_u128 / (NANOS_PER_SEC as u128)) as u64,
@@ -588,16 +588,16 @@ impl Div<f64> for Duration {
 
     fn div(self, rhs: f64) -> Duration {
         const NPS: f64 = NANOS_PER_SEC as f64;
-        if rhs.is_sign_negative() {
-            panic!("duration can not be divided by negative float");
-        }
         let nanos_f64 = (NPS * (self.secs as f64) + (self.nanos as f64)) / rhs;
         if !nanos_f64.is_finite() {
             panic!("got non-finite value when dividing duration by float");
         }
         if nanos_f64 > MAX_NANOS_F64 {
             panic!("overflow when dividing duration by float");
-        };
+        }
+        if nanos_f64 < 0.0 {
+            panic!("underflow when multiplying duration by float");
+        }
         let nanos_u128 = nanos_f64 as u128;
         Duration {
             secs: (nanos_u128 / (NANOS_PER_SEC as u128)) as u64,