In a DateTimeMaskedInput with Mask=hh:mm tt, if the value is 10:00, the user can't enter 04:00.
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Reason for decline: The pattern "hh:mm tt" cannot accept time of type "00:00". You can test this with DateTime.ToString() method which will return "12:00" if you give him time (0hours, 0 minutes) and pattern "hh:mm tt". When typing 0 in the begging of the MaskedDateTimeInput , the control automatically converts 00:00 to 12:00 which is expected.

Workaround: You can use Mask="HH:mm tt" or Mask="t".