Welcome to Visual WebGUi
I hope I understand you correctly, that you want to show a DateTimePicker control inside of a DataGridView cell ?
First to note is the inevitable platform difference between a desktop (Windows Forms) application and a web (Visual WebGui) application. You can see more on this subject here. In short, and related to your case, the Windows Forms's drawing and graphical methods are not implemented in Visual WebGui so you will need to use another approach for this customization.
There are two kinds of approaches that you can take for this customizations. First, you can define your own custom column type and we have a sample of a custom combobox column in our CompanionKit here. Second, each cell of a DataGridView has an initially hidden Panel associated with it, which can be made visible and will in that case be shown in place of the standard cell contents. The panel option is more powerful in the sense that you can have the panel span multiple columns and rows, but a custom column type may be more convenient to use. More info on the DataGridViewCellPanel can be found here and more info on the DataGridView here.
Hope this helps,