time: wrong Sign Cast in Time.UnmarshalBinary (V2 Format)

Fixes #78528

Author: cuiweixie

src/time/time.go

@@ -1552,7 +1552,7 @@ func (t *Time) UnmarshalBinary(data []byte) error {
 	buf = buf[4:]
 	offset := int(int16(buf[1])|int16(buf[0])<<8) * 60
 	if version == timeBinaryVersionV2 {
-		offset += int(buf[2])
+		offset += int(int8(buf[2]))
 	}
 
 	*t = Time{}

src/time/time_test.go

@@ -1752,6 +1752,29 @@ func TestMarshalBinaryVersion2(t *testing.T) {
 	}
 }
 
+func TestMarshalBinaryVersion2Bugfix(t *testing.T) {
+	offsetSec := -5*3600 - 30*60 - 45 // -19845
+	loc := FixedZone("LMT", offsetSec)
+
+	t1 := Date(2024, 6, 15, 12, 0, 0, 0, loc)
+	b, err := t1.MarshalBinary()
+	if err != nil {
+		t.Errorf("Failed to Marshal, error = %v", err)
+	}
+
+	t2 := Time{}
+	err = t2.UnmarshalBinary(b)
+	if err != nil {
+		t.Errorf("Failed to Unmarshal, error = %v", err)
+	}
+
+	_, t1OriginOffset := t1.Zone()
+	_, t2OriginOffset := t2.Zone()
+	if t1OriginOffset != t2OriginOffset {
+		t.Errorf("The result offsetSec: %d after Unmarshal is not matched original offsetSec: %d", t2OriginOffset, t1OriginOffset)
+	}
+}
+
 func TestUnmarshalTextAllocations(t *testing.T) {
 	in := []byte(testdataRFC3339UTC) // short enough to be stack allocated
 	if allocs := testing.AllocsPerRun(100, func() {