Replace this:
DateTime y = Convert.ToDateTime(tanggal);
with this:
DateTime y = DateTime.ParseExact(tanggal, "dd/MM/yy H:mm", CultureInfo.InvariantCulture);
Note: I'm not sure what value corresponds to month and day. Please, change it accordingly.
For further details, please see:
DateTime.ParseExact Method (System) | Microsoft Docs[
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