diff --git a/server/internal/db/migrate.go b/server/internal/db/migrate.go
index c04319b..105bd6f 100644
--- a/server/internal/db/migrate.go
+++ b/server/internal/db/migrate.go
@@ -120,13 +120,21 @@ func switchoverAt(ctx context.Context) (time.Time, error) {
 		return time.Time{}, err
 	}
 	var raw sql.NullString
-	if err := DB.QueryRowContext(ctx, `SELECT MIN(read_at) FROM mail_reads`).Scan(&raw); err != nil {
+	// CAST 成文本再解析：两种方言的驱动对 timestamptz 返回的类型不一样
+	// （PG 给 time.Time，SQLite 给字符串），文本形态才是共通的。
+	if err := DB.QueryRowContext(ctx, `SELECT CAST(MIN(read_at) AS TEXT) FROM mail_reads`).Scan(&raw); err != nil {
 		return time.Time{}, err
 	}
 	if !raw.Valid {
 		return time.Now().UTC(), nil // 表还是空的：本机没有"老邮件"要照顾
 	}
-	for _, layout := range []string{"2006-01-02 15:04:05.000", "2006-01-02 15:04:05", time.RFC3339} {
+	// 认不出来的形态退回"现在"：那等于把当下已有的已读邮件全算作迁移前 ——
+	// 首次升级时正是对的；而且这条路有一次性标记守着，不会反复跑。
+	for _, layout := range []string{
+		"2006-01-02 15:04:05.000", "2006-01-02 15:04:05",
+		"2006-01-02 15:04:05.999999-07", "2006-01-02 15:04:05-07",
+		time.RFC3339,
+	} {
 		if t, perr := time.Parse(layout, raw.String); perr == nil {
 			return t.UTC(), nil
 		}