When grouping by date/time values, only the date portion of the value is taken into account by default, while the time portion is ignored. So rows that contain matching date portions, but different time portions, in a grouping column are combined into a single group. If grouping columns contain other data types, the default grouping logic is used. In this instance, rows are combined into a single group if they have a matching value in a grouping column.
The Interval Grouping feature allows you to change the default logic, especially for columns containing date/time values. For example, data rows can be arranged by the month or year portion of a date/time value, while text columns can be grouped alphabetically by the first characters. To specify the required grouping mode, use the column's GridColumnBase.GroupInterval property.