diff --git a/README.md b/README.md
index d3d20a4..bbab1d4 100644
--- a/README.md
+++ b/README.md
@@ -469,6 +469,7 @@ from 'ansi.json' FILTER 'colors' AS src;
    *from* and *to*. The *diff* specifies the units in which the
    difference is computed:
    - `year`, `yy`, `yyyy`: difference between date year parts
+   - `month`, `mm`, `m`:     difference in months
    - `day`, `dd`, `d`: difference in calendar days
    - `hour`, `hh`: difference in hours
    - `minute`, `mi`, `n`: difference in minutes
diff --git a/lang/builtin.go b/lang/builtin.go
index a826502..6e05aca 100644
--- a/lang/builtin.go
+++ b/lang/builtin.go
@@ -1121,7 +1121,8 @@ func builtInDateDiff(args []Expr, row *Row, rows []*Row) (types.Value, error) {
 		// XXX quarter, qq, q
 
 	case "month", "mm", "m":
-		return types.IntValue(to.Month() - from.Month()), nil
+		return types.IntValue((int64(to.Year())*12 + int64(to.Month())) -
+			(int64(from.Year())*12 + int64(from.Month()))), nil
 
 		// XXX dayofyear, dy, y
 
diff --git a/lang/builtin_test.go b/lang/builtin_test.go
index 324d687..c715327 100644
--- a/lang/builtin_test.go
+++ b/lang/builtin_test.go
@@ -1,5 +1,5 @@
 //
-// Copyright (c) 2020-2021 Markku Rossi
+// Copyright (c) 2020-2023 Markku Rossi
 //
 // All rights reserved.
 //
@@ -446,6 +446,18 @@ SELECT UNICODE(nstring), NCHAR(UNICODE(nstring));`,
                             '2006-01-01 00:00:00.0000000');`,
 		v: [][]string{{"1"}},
 	},
+	{
+		q: `SELECT DATEDIFF(month,
+                            '2005-12-31 23:59:59.9999999',
+                            '2006-01-01 00:00:00.0000000');`,
+		v: [][]string{{"1"}},
+	},
+	{
+		q: `SELECT DATEDIFF(month,
+                            '2005-01-01 00:00:00.0000000',
+                            '2006-02-01 00:00:00.0000000');`,
+		v: [][]string{{"13"}},
+	},
 	{
 		q: `SELECT DATEDIFF(day,
                             '2005-12-31 23:59:59.9999999',