This collection is used to express a group of dates or times which are used in an AutoFilter criteria. Values are always written in the calendar type of the first date encountered in the filter range, so that all subsequent dates, even when formatted or represented by other calendar types, can be correctly compared for the purposes of filtering.
Creates a new DateGroupItem instanciating the object filter should apply for grouping
Grouping level This must be one of year, month, day, hour, minute or second.
Serialize the object to xml
Year (4 digits)