Since some months of the year have a different number of days than others, and leap years have more days than "normal" years, it is not always possible to know the exact amount of time spanned by an interval without providing a specific date as a context. The NormalSpan property provides an estimation of this by normalizing months and years; for the purposes of sorting the members of the AdditionalIntervals collection, and determining which ones can appear, all DateInterval instances whose IntervalUnits property is set to 'Months' are assumed to contain 30 days, and all instances whose IntervalUnits property is set to 'Years' are assumed to have 365 days.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Server 2012, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2