Add special behavior for extending

Author: pohuan

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 selection includes last two days of month, it will use "extension" semantics and will includes day till end of month. For example, when Feb 27 and 28 of 2013 are included and a month is added, it will get March 27 to 31.
+
+This behavior will only happen when last two days of month are included. 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)