It's likely because you are trying to set the text value of calSendDate
to value
regardless if it's null or not.
Instead, change your property setter to this:
Set(value As Nullable(Of DateTime))
If value.HasValue Then
calSendDate.Text = value
Else
calSendDate.Text = Nothing
End If
End Set
Additional advice:
If your project is .NET 3.5 or above and references System.Data.DataSetExtensions.dll, change your first block of code to this instead:
ctrl_prov_dtl.SelectedDateSendDate = dr.Field(Of DateTime?)("SendDate")
Also, your property getter never returns the parsed myDateTime
value. And returning 1/1/1900 as the fallback value on a nullable type feels very wrong to me.