diff --git a/plan/typeinferer.go b/plan/typeinferer.go
index b713d3270d14d..a673908d2bb6c 100644
--- a/plan/typeinferer.go
+++ b/plan/typeinferer.go
@@ -172,7 +172,7 @@ func (v *typeInferrer) binaryOperation(x *ast.BinaryOperationExpr) {
 		x.Type.Init(mysql.TypeLonglong)
 	case opcode.Plus, opcode.Minus, opcode.Mul, opcode.Mod:
 		if x.L.GetType() != nil && x.R.GetType() != nil {
-			xTp := mergeArithType(x.L.GetType().Tp, x.R.GetType().Tp)
+			xTp := mergeArithType(x.L.GetType(), x.R.GetType())
 			x.Type.Init(xTp)
 			leftUnsigned := x.L.GetType().Flag & mysql.UnsignedFlag
 			rightUnsigned := x.R.GetType().Flag & mysql.UnsignedFlag
@@ -181,7 +181,7 @@ func (v *typeInferrer) binaryOperation(x *ast.BinaryOperationExpr) {
 		}
 	case opcode.Div:
 		if x.L.GetType() != nil && x.R.GetType() != nil {
-			xTp := mergeArithType(x.L.GetType().Tp, x.R.GetType().Tp)
+			xTp := mergeArithType(x.L.GetType(), x.R.GetType())
 			if xTp == mysql.TypeLonglong {
 				xTp = mysql.TypeNewDecimal
 			}
@@ -192,7 +192,21 @@ func (v *typeInferrer) binaryOperation(x *ast.BinaryOperationExpr) {
 	x.Type.Collate = charset.CollationBin
 }
 
-func mergeArithType(a, b byte) byte {
+// toArithType converts DateTime, Duration and Timestamp types to NewDecimal type if Decimal > 0.
+func toArithType(ft *types.FieldType) (tp byte) {
+	tp = ft.Tp
+	if types.IsTypeTime(tp) {
+		if ft.Decimal > 0 {
+			tp = mysql.TypeNewDecimal
+		} else {
+			tp = mysql.TypeLonglong
+		}
+	}
+	return
+}
+
+func mergeArithType(fta, ftb *types.FieldType) byte {
+	a, b := toArithType(fta), toArithType(ftb)
 	switch a {
 	case mysql.TypeString, mysql.TypeVarchar, mysql.TypeVarString, mysql.TypeDouble, mysql.TypeFloat:
 		return mysql.TypeDouble
@@ -220,7 +234,7 @@ func (v *typeInferrer) unaryOperation(x *ast.UnaryOperationExpr) {
 		x.Type.Init(mysql.TypeLonglong)
 		if x.V.GetType() != nil {
 			switch x.V.GetType().Tp {
-			case mysql.TypeString, mysql.TypeVarchar, mysql.TypeVarString, mysql.TypeDouble, mysql.TypeFloat:
+			case mysql.TypeString, mysql.TypeVarchar, mysql.TypeVarString, mysql.TypeDouble, mysql.TypeFloat, mysql.TypeDatetime, mysql.TypeDuration, mysql.TypeTimestamp:
 				x.Type.Tp = mysql.TypeDouble
 			case mysql.TypeNewDecimal:
 				x.Type.Tp = mysql.TypeNewDecimal
@@ -279,7 +293,7 @@ func (v *typeInferrer) handleFuncCallExpr(x *ast.FuncCallExpr) {
 		if len(x.Args) > 0 {
 			tp = x.Args[0].GetType()
 			for i := 1; i < len(x.Args); i++ {
-				mergeArithType(tp.Tp, x.Args[i].GetType().Tp)
+				mergeArithType(tp, x.Args[i].GetType())
 			}
 		}
 	case "interval":
diff --git a/plan/typeinferer_test.go b/plan/typeinferer_test.go
index b365d4262d896..7e46a333b822f 100644
--- a/plan/typeinferer_test.go
+++ b/plan/typeinferer_test.go
@@ -41,7 +41,7 @@ func (ts *testTypeInferrerSuite) TestInferType(c *C) {
 	defer store.Close()
 	testKit := testkit.NewTestKit(c, store)
 	testKit.MustExec("use test")
-	testKit.MustExec("create table t (c1 int, c2 double, c3 text)")
+	testKit.MustExec("create table t (c1 int, c2 double, c3 text, c4 timestamp)")
 	cases := []struct {
 		expr string
 		tp   byte
@@ -51,6 +51,8 @@ func (ts *testTypeInferrerSuite) TestInferType(c *C) {
 		{"+1", mysql.TypeLonglong, charset.CharsetBin},
 		{"-1", mysql.TypeLonglong, charset.CharsetBin},
 		{"-'1'", mysql.TypeDouble, charset.CharsetBin},
+		{"-curtime()", mysql.TypeDouble, charset.CharsetBin},
+		{"-now()", mysql.TypeDouble, charset.CharsetBin},
 		{"~1", mysql.TypeLonglong, charset.CharsetBin},
 		{"1e0", mysql.TypeDouble, charset.CharsetBin},
 		{"1.0", mysql.TypeNewDecimal, charset.CharsetBin},
@@ -73,6 +75,22 @@ func (ts *testTypeInferrerSuite) TestInferType(c *C) {
 
 		{"1 + '1'", mysql.TypeDouble, charset.CharsetBin},
 		{"1 + 1.1", mysql.TypeNewDecimal, charset.CharsetBin},
+		{"now() + 0", mysql.TypeLonglong, charset.CharsetBin},
+		{"curtime() + 0", mysql.TypeLonglong, charset.CharsetBin},
+		{"now(0) + 0", mysql.TypeLonglong, charset.CharsetBin},
+		{"now(2) + 0", mysql.TypeNewDecimal, charset.CharsetBin},
+		{"now() + 1.1", mysql.TypeNewDecimal, charset.CharsetBin},
+		{"now() + '1'", mysql.TypeDouble, charset.CharsetBin},
+		{"now(2) + '1'", mysql.TypeDouble, charset.CharsetBin},
+		{"now() + curtime()", mysql.TypeLonglong, charset.CharsetBin},
+		{"now() + now()", mysql.TypeLonglong, charset.CharsetBin},
+		{"now() + now(2)", mysql.TypeNewDecimal, charset.CharsetBin},
+		{"c2 + now()", mysql.TypeDouble, charset.CharsetBin},
+		{"c4 + 1", mysql.TypeLonglong, charset.CharsetBin},
+		{"c4 + 1.1", mysql.TypeNewDecimal, charset.CharsetBin},
+		{"c4 + '1.1'", mysql.TypeDouble, charset.CharsetBin},
+		{"1.1 + now()", mysql.TypeNewDecimal, charset.CharsetBin},
+		{"1 + now()", mysql.TypeLonglong, charset.CharsetBin},
 		{"1 div 2", mysql.TypeLonglong, charset.CharsetBin},
 		{"1 / 2", mysql.TypeNewDecimal, charset.CharsetBin},
 
@@ -106,6 +124,7 @@ func (ts *testTypeInferrerSuite) TestInferType(c *C) {
 		{"curtime()", mysql.TypeDuration, charset.CharsetBin},
 		{"current_time()", mysql.TypeDuration, charset.CharsetBin},
 		{"curtime()", mysql.TypeDuration, charset.CharsetBin},
+		{"curtime(2)", mysql.TypeDuration, charset.CharsetBin},
 		{"current_timestamp()", mysql.TypeDatetime, charset.CharsetBin},
 		{"utc_timestamp()", mysql.TypeDatetime, charset.CharsetBin},
 		{"microsecond('2009-12-31 23:59:59.000010')", mysql.TypeLonglong, charset.CharsetBin},
diff --git a/util/types/etc.go b/util/types/etc.go
index c9439c8d90586..cc17fa73855da 100644
--- a/util/types/etc.go
+++ b/util/types/etc.go
@@ -49,6 +49,17 @@ func IsTypeChar(tp byte) bool {
 	}
 }
 
+// IsTypeTime returns a boolean indicating
+// whether the tp is the time type like a datetime type, a duration type, or a timestamp type.
+func IsTypeTime(tp byte) bool {
+	switch tp {
+	case mysql.TypeDatetime, mysql.TypeDuration, mysql.TypeTimestamp:
+		return true
+	default:
+		return false
+	}
+}
+
 // IsTypePrefixable returns a boolean indicating
 // whether an index on a column with the tp can be defined with a prefix.
 func IsTypePrefixable(tp byte) bool {