When you select a particular cell and if the cell is set to Editable
then the cell's editor is shown. To use a particular cell editor, you have to add these lines of code :
// aDateColumn is the column you want to format
// Custom mask
RepositoryItemTextEdit columnEdit = new RepositoryItemTextEdit();
columnEdit.Mask.MaskType = DevExpress.XtraEditors.Mask.MaskType.DateTime;
columnEdit.Mask.EditMask = "d";
aDateColumn.ColumnEdit = columnEdit;
More info here.