Time transform expression function accepts strings also

pkg/expr/func_time.go

@@ -173,6 +173,14 @@ func prepMod(base interface{}, mod interface{}) (*time.Time, int, error) {
 		t = &auxt
 	case *time.Time:
 		t = auxt
+	case string:
+		tt, err := cast.ToTimeE(auxt)
+
+		if err != nil {
+			return nil, 0, err
+		}
+
+		t = &tt
 	default:
 		return nil, 0, errors.New("unexpected input type")
 	}

pkg/expr/func_time_test.go

@@ -37,6 +37,13 @@ func Example_strftimeWithModTime() {
 	// 1993-02-02T06:30:00
 }
 
+func Example_strftimeWithStringValue() {
+	eval(`strftime("1993-02-02T06:30:00", "%Y-%m-%dT%H:%M:%S")`, exampleTimeParams)
+
+	// output:
+	// 1993-02-02T06:30:00
+}
+
 func Example_parseISODate() {
 	eval(`date("1993-02-02T06:00:00-05:00")`, nil)