diff --git a/umadump-gametora.go b/umadump-gametora.go
index 610a7c4..e31d559 100644
--- a/umadump-gametora.go
+++ b/umadump-gametora.go
@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/json"
 	"flag"
+	"fmt"
 	"io"
 	"log/slog"
 	"net/http"
@@ -88,12 +89,13 @@ func filejson[T any](path string) ([]T, error) {
 }
 
 func main() {
+	ts := time.Now().UTC().Format(time.DateOnly) // can't use RFC3339 because :s in filename
 	var (
 		dumpDir string
 		out     string
 	)
 	flag.StringVar(&dumpDir, "dump", ".", "`dir`ectory containing umadump output files")
-	flag.StringVar(&out, "o", "", "output `file` (default stdout)")
+	flag.StringVar(&out, "o", fmt.Sprintf("uma-col-all-%s.json", ts), "output `file` (stdout if empty)")
 	flag.Parse()
 
 	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt)
@@ -142,7 +144,7 @@ func main() {
 		Game:      "umamusume",
 		Type:      "collection",
 		Version:   4,
-		Timestamp: time.Now().String(),
+		Timestamp: ts,
 		Servers: map[string]CollectionServer{
 			"en": {
 				CharCards: charaLevels,