diff --git a/Libraries/LibJS/Runtime/Temporal/Duration.cpp b/Libraries/LibJS/Runtime/Temporal/Duration.cpp
index 3e667886616..0e3eb7292f7 100644
--- a/Libraries/LibJS/Runtime/Temporal/Duration.cpp
+++ b/Libraries/LibJS/Runtime/Temporal/Duration.cpp
@@ -734,7 +734,7 @@ ThrowCompletionOr<TimeDuration> add_24_hour_days_to_time_duration(VM& vm, TimeDu
 }
 
 // 7.5.24 AddTimeDurationToEpochNanoseconds ( d, epochNs ), https://tc39.es/proposal-temporal/#sec-temporal-addtimedurationtoepochnanoseconds
-TimeDuration add_time_duration_to_epoch_nanoseconds(TimeDuration const& duration, TimeDuration const& epoch_nanoseconds)
+Crypto::SignedBigInteger add_time_duration_to_epoch_nanoseconds(TimeDuration const& duration, Crypto::SignedBigInteger const& epoch_nanoseconds)
 {
     // 1. Return epochNs + ℤ(d).
     return epoch_nanoseconds.plus(duration);
diff --git a/Libraries/LibJS/Runtime/Temporal/Duration.h b/Libraries/LibJS/Runtime/Temporal/Duration.h
index c0c6507bf32..8cf4b14c11e 100644
--- a/Libraries/LibJS/Runtime/Temporal/Duration.h
+++ b/Libraries/LibJS/Runtime/Temporal/Duration.h
@@ -129,7 +129,7 @@ GC::Ref<Duration> create_negated_temporal_duration(VM&, Duration const&);
 TimeDuration time_duration_from_components(double hours, double minutes, double seconds, double milliseconds, double microseconds, double nanoseconds);
 ThrowCompletionOr<TimeDuration> add_time_duration(VM&, TimeDuration const&, TimeDuration const&);
 ThrowCompletionOr<TimeDuration> add_24_hour_days_to_time_duration(VM&, TimeDuration const&, double days);
-TimeDuration add_time_duration_to_epoch_nanoseconds(TimeDuration const& duration, TimeDuration const& epoch_nanoseconds);
+Crypto::SignedBigInteger add_time_duration_to_epoch_nanoseconds(TimeDuration const& duration, Crypto::SignedBigInteger const& epoch_nanoseconds);
 i8 compare_time_duration(TimeDuration const&, TimeDuration const&);
 TimeDuration time_duration_from_epoch_nanoseconds_difference(Crypto::SignedBigInteger const&, Crypto::SignedBigInteger const&);
 ThrowCompletionOr<TimeDuration> round_time_duration_to_increment(VM&, TimeDuration const&, Crypto::UnsignedBigInteger const& increment, RoundingMode);
diff --git a/Libraries/LibJS/Runtime/Temporal/ZonedDateTimePrototype.cpp b/Libraries/LibJS/Runtime/Temporal/ZonedDateTimePrototype.cpp
index fe480a1a652..f9d986cfc9a 100644
--- a/Libraries/LibJS/Runtime/Temporal/ZonedDateTimePrototype.cpp
+++ b/Libraries/LibJS/Runtime/Temporal/ZonedDateTimePrototype.cpp
@@ -703,8 +703,8 @@ JS_DEFINE_NATIVE_FUNCTION(ZonedDateTimePrototype::round)
         // i. Let roundedDayNs be ! RoundTimeDurationToIncrement(dayProgressNs, dayLengthNs, roundingMode).
         auto rounded_day_nanoseconds = MUST(round_time_duration_to_increment(vm, day_progress_nanoseconds, day_length_nanoseconds.unsigned_value(), rounding_mode));
 
-        // j. Let epochNanoseconds be AddTimeDurationToEpochNanoseconds(startNs, roundedDayNs).
-        epoch_nanoseconds = add_time_duration_to_epoch_nanoseconds(start_nanoseconds, rounded_day_nanoseconds);
+        // j. Let epochNanoseconds be AddTimeDurationToEpochNanoseconds(roundedDayNs, startNs).
+        epoch_nanoseconds = add_time_duration_to_epoch_nanoseconds(rounded_day_nanoseconds, start_nanoseconds);
     }
     // 19. Else,
     else {