protected IEnumerable <DateInterval> EnumIntervals()
{
var intervals = Interval.EnumMonths().ToArray();
for (int i = (intervals.Length - 1) % GroupMonths + 1 - GroupMonths; i < intervals.Length; i += GroupMonths)
{
yield return(new DateInterval(intervals[Math.Max(0, i)].Start, intervals[i + GroupMonths - 1].End));
}
}