I have run into this in the past. While you can send mouse or keyboard messages to resolve this, it's a little easier if you don't mind using what is an ugly hack, but one that works.
var format = dateTimePicker1.Format;
dateTimePicker1.Format = DateTimePickerFormat.Custom;
dateTimePicker1.Format = format;
The trick is to force the control to internally reset itself by changing the
Format
and then setting it back to what it was. The code above assumes that it is something other than
Custom
.