You can use this:
DateTime.Now.ToString("dd-MM-yy.hh.mm")
If you want 24 hours:
DateTime.Now.ToString("dd-MM-yy.hh.mm")
See the MSDN documentation.
Please note that the format string that you first used is incorrect, since mm stands for minutes, so you should have used mm for several months.
You also use Today , which does not have a time component (so it will always be 00:00:00). If you need those (hours, minutes, seconds and milliseconds), you should use Now .
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