fix datetime json MarshalJSON

Author: ti

bson/primitive/primitive.go

@@ -10,7 +10,9 @@ package primitive // import "go.mongodb.org/mongo-driver/bson/primitive"
 
 import (
 	"bytes"
+	"encoding/json"
 	"fmt"
+	"time"
 )
 
 // Binary represents a BSON binary value.
@@ -33,6 +35,11 @@ type Undefined struct{}
 // DateTime represents the BSON datetime value.
 type DateTime int64
 
+// MarshalJSON marshal to time type 
+func (d DateTime) MarshalJSON() ([]byte, error) {
+	return json.Marshal(time.Unix(int64(d)/1000, int64(d)%1000*1000000))
+}
+
 // Null repreesnts the BSON null value.
 type Null struct{}