diff --git a/src/time/export_test.go b/src/time/export_test.go
index f4a8cd9b72a..0f2d21053a3 100644
--- a/src/time/export_test.go
+++ b/src/time/export_test.go
@@ -51,6 +51,7 @@ const (
 	RuleJulian       = RuleKind(ruleJulian)
 	RuleDOY          = RuleKind(ruleDOY)
 	RuleMonthWeekDay = RuleKind(ruleMonthWeekDay)
+	UnixToInternal   = unixToInternal
 )
 
 type Rule struct {
diff --git a/src/time/internal_test.go b/src/time/internal_test.go
index ffe54e47c2d..87a4208b058 100644
--- a/src/time/internal_test.go
+++ b/src/time/internal_test.go
@@ -62,4 +62,6 @@ func CheckRuntimeTimerPeriodOverflow() {
 var (
 	MinMonoTime = Time{wall: 1 << 63, ext: -1 << 63, loc: UTC}
 	MaxMonoTime = Time{wall: 1 << 63, ext: 1<<63 - 1, loc: UTC}
+
+	NotMonoNegativeTime = Time{wall: 0, ext: -1<<63 + 50}
 )
diff --git a/src/time/time.go b/src/time/time.go
index 7e5192a0c97..841f989293c 100644
--- a/src/time/time.go
+++ b/src/time/time.go
@@ -189,8 +189,15 @@ func (t *Time) addSec(d int64) {
 		t.stripMono()
 	}
 
-	// TODO: Check for overflow.
-	t.ext += d
+	// Check if the sum of t.ext and d overflows and handle it properly.
+	sum := t.ext + d
+	if (sum > t.ext) == (d > 0) {
+		t.ext = sum
+	} else if d > 0 {
+		t.ext = 1<<63 - 1
+	} else {
+		t.ext = -(1<<63 - 1)
+	}
 }
 
 // setLoc sets the location associated with the time.
diff --git a/src/time/time_test.go b/src/time/time_test.go
index 8884731e1d0..3a58bfe4e94 100644
--- a/src/time/time_test.go
+++ b/src/time/time_test.go
@@ -1481,7 +1481,7 @@ func TestTimeIsDST(t *testing.T) {
 	tzFixed := FixedZone("FIXED_TIME", 12345)
 
 	tests := [...]struct {
-		time   Time
+		time Time
 		want bool
 	}{
 		0: {Date(2009, 1, 1, 12, 0, 0, 0, UTC), false},
@@ -1501,3 +1501,28 @@ func TestTimeIsDST(t *testing.T) {
 		}
 	}
 }
+
+func TestTimeAddSecOverflow(t *testing.T) {
+	// Test it with positive delta.
+	var maxInt64 int64 = 1<<63 - 1
+	timeExt := maxInt64 - UnixToInternal - 50
+	notMonoTime := Unix(timeExt, 0)
+	for i := int64(0); i < 100; i++ {
+		sec := notMonoTime.Unix()
+		notMonoTime = notMonoTime.Add(Duration(i * 1e9))
+		if newSec := notMonoTime.Unix(); newSec != sec+i && newSec+UnixToInternal != maxInt64 {
+			t.Fatalf("time ext: %d overflows with positive delta, overflow threshold: %d", newSec, maxInt64)
+		}
+	}
+
+	// Test it with negative delta.
+	maxInt64 = -maxInt64
+	notMonoTime = NotMonoNegativeTime
+	for i := int64(0); i > -100; i-- {
+		sec := notMonoTime.Unix()
+		notMonoTime = notMonoTime.Add(Duration(i * 1e9))
+		if newSec := notMonoTime.Unix(); newSec != sec+i && newSec+UnixToInternal != maxInt64 {
+			t.Fatalf("time ext: %d overflows with positive delta, overflow threshold: %d", newSec, maxInt64)
+		}
+	}
+}