diff --git a/js/src/builtin/temporal/PlainDateTime.cpp b/js/src/builtin/temporal/PlainDateTime.cpp
index a708fb8fb0e2df..5581b8a1d26504 100644
--- a/js/src/builtin/temporal/PlainDateTime.cpp
+++ b/js/src/builtin/temporal/PlainDateTime.cpp
@@ -714,6 +714,7 @@ static bool AddDateTime(JSContext* cx, const PlainDateTime& dateTime,
   // Step 4.
   Duration dateDuration = {duration.years, duration.months, duration.weeks,
                            daysResult};
+  MOZ_ASSERT(IsValidDuration(duration));
 
   // Step 5.
   PlainDate addedDate;
diff --git a/js/src/builtin/temporal/PlainTime.cpp b/js/src/builtin/temporal/PlainTime.cpp
index 4c2ed62656c9b8..51d3ef21d3d4b1 100644
--- a/js/src/builtin/temporal/PlainTime.cpp
+++ b/js/src/builtin/temporal/PlainTime.cpp
@@ -1482,7 +1482,15 @@ static bool AddTimeDaysSlow(JSContext* cx, const PlainTime& time,
     return false;
   }
 
-  *result = BigInt::numberValue(days);
+  // The days number is used as the input for a duration. Throw if the BigInt
+  // when converted to a Number can't be represented in a duration.
+  double daysNumber = BigInt::numberValue(days);
+  if (!ThrowIfInvalidDuration(cx, {0, 0, 0, daysNumber})) {
+    return false;
+  }
+  MOZ_ASSERT(IsInteger(daysNumber));
+
+  *result = daysNumber;
   return true;
 }
 
@@ -1635,6 +1643,7 @@ bool js::temporal::AddTime(JSContext* cx, const PlainTime& time,
   if (!AddTimeDays(cx, time, duration, &days)) {
     return false;
   }
+  MOZ_ASSERT(IsInteger(days));
 
   *result = balanced;
   *daysResult = days;