Merge pull request #809 from pohuan/dev/powei/add-statement-dateadd

Add dateAdd behavior description of extension semantics

Author: prmerger-automator[bot]

query-languages/dax/dateadd-function-dax.md

@@ -57,6 +57,16 @@ The following formula calculates dates that are one year before the dates in the
 = DATEADD(DateTime[DateKey],-1,year)
 ```
 
+## Special behavior
+
+When the selection includes the last two days of month, DATEADD will use "extension" semantics and will include the days till the end of month. For example, when Feb 27 and 28 of 2013 are included in the selection and a month is added, DATEADD will return March 27 to 31.
+
+This behavior only happens when last two days of month are included in the selection. If only Feb 27 is selected, it will go to March 27.
+
+```dax
+= DATEADD(DateTime[DateKey], 1, month)
+```
+
 ## Related content
 
 [Time intelligence functions](time-intelligence-functions-dax.md)