End of month boundaries: end date 29 through 31
If the end date is 29 through 31, then the end dates of the each previous month will be the same corresponding day of the month unless a month is short and lacks that day of the month. In that case the end date for the short month will be the last calendar day of the month in that month.
If a month end date reverts to the last day because the month is short of days, then the month end of the month previous to the shortened month will recover the lost end days. This behavior holds true in the trend within monthly subgroups of rolling monthly periods as well as in trends of a series of months.
For example, if the end date is 3/31, then the end dates of previous months will be 2/28, 1/31, 12/31, 11/30, 10/31, etc.
These monthly end dates are preserved even if they are embedded in a trend period that ends on 30. For example, if the trend has four three-month time periods in it, and the end date of the last period is 3/31, then going backward the end dates of the four trend periods will be 3/31, 12/31, 9/30, and 6/30. If these periods are subdivided into monthly subgroupings, then the one ending 9/30 will have three monthly subgroups ending 7/31, 8/31, and 9/30. Note that the July and August subgroups end on 31 even though they belong to a period that ends on 30. The reason for this is that the end date of the subgroups is governed by the 3/31 end date of the last trend period and not the 9/30 end date of the period.
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