When the Range object represents a single cell in a PivotTable field's data range, the Group method performs numeric or date-based grouping in that field.
The Boolean array for the Periods parameter contains the following elements:
The following arguments are optional
Start (Long) - The first value to be grouped. If this argument is omitted or True, the first value in the field is used.
End - The last value to be grouped. If this argument is omitted or True, the last value in the field is used
By (Integer) - If the field is numeric, this argument specifies the size of each group. If the field is a date, this argument specifies the number of days in each group if element 4 in the Periods array is True and all the other elements are False. Otherwise, this argument is ignored. If this argument is omitted, Microsoft Excel automatically chooses a default group size.
Periods (Boolean) - An array of Boolean values that specify the period for the group, described in the Remarks section. If an element in the array is True, a group is created for the corresponding time; if the element is False, no group is created. If the field isn't a date field, this argument is ignored.