By default, the GridDataBoundGrid places a MonthCalendar cell control if you column has datatype of DateTime. And, this control does not allow you to set the time.
But you can see the time by providing the proper GridBoundColumn.StyleInfo.Format property that includes the time. This sample shows this formatting and hides the button so the user cannot drop the calendar and kill the time.
Forum_DateTime_3465.zip
If you want to use a DateImePicker in the cell (instead of the deafult MonthCalendar), here is a KB article that contains a custom cell type that does this.
http://www.syncfusion.com/Support/article.aspx?id=10435