If you are going to use the xamGrid in your application you have to reference not only the InfragisticsWPF.Controls.Grids.XamGrid.v11.2 assembly, but you have to add the InfragisticsWPF.Controls.Menus.XamMenu.v11.2 and the InfragisticsWPF.DataManager.v11.2 assemblies too.
If you use a DateColumn, your default editor will be the DatePicker, which will remove the time part of the date and will just maintain the date. If you want to preserve the time, you should use the DateTimeColumn instead.