Indeed the pattern you need in order to set the value of the DateTimePicker is the ValuePattern
and so far we are not aware of any issues similar to the one you are facing.
So we have tested it with the UI Spy
tool and with the sample code you have provided (as shown in the attached project), but we were not able to reproduce the exception.
Therefore it would be great if you can check out the project and let me know if I have missed something. If so it would be helpful if you can send us a sample project that demonstrates the exception.
Looking forward to your replay.
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