Amazing command that dateadd having datepart second, month, etc. Need to know what you are doing when you use it...
From the explanation, it says that:
If datepart is month and the date month has more days than the return month and the date day does not exist in the return month, the last day of the return month is returned. For example, September has 30 days; therefore, the two following statements return 2006-09-30 00:00:00.000:
SELECT DATEADD(month, 1, '2006-08-30')
SELECT DATEADD(month, 1, '2006-08-31')
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