Hi,
I am not very sure about the reason why are you using NumberFormat for the date types. Basically, C1DateEdit is used to format the date values. You are suggested to try setting the CustomFormat of the C1DateEdit to 'MM/dd/yyyy’ and check if the problem persists.
Please note, there is no need to set the NumberFormat of the C1TrueDBGrid’s column as you are already assigning a custom editor (i.e. C1DateEdit) to the column.
However, I have tried using the exact same code as yours and the problem does not replicate at my end. Please find a small sample application implementing the same attached.
Since you are using a very older build of C1TrueDBGrid, I would also request you to try using the latest build as it includes a lot more bug fixes and new features. The latest build can be downloaded from the following link:
http://prerelease.componentone.com/dotnet20/c1truedbgrid/2013-t3/C1TrueDBGrid.2_2.0.20133.61336.zip
Thanks and Regards,
Pragati Kaushik