The topic for datediff fails to mention that SET DATEFIRST does not apply by Erland Sommarskog

It is not unreasonable to expect that datediff(week, date1, date2) would respect the setting of SET DATEFIRST. However this is not the case, because that would make the function nondeterministic as discussed in the response to item
https://connect.microsoft.com/SQLServer/feedback/details/239958/datediff-function-and-the-first-day-of-the-week#details

There is no reason to change this, but since users may expect a different behaviour, there is all reason to explicitly point out in the topics for datediff() and SET DATEFIRST that the DATEFIRST setting is not applicable to datediff.