Return WeekNo (ISO 8601 standard)

In MS SQL, January 1 of any year defines the starting number for the week. The DATEPART(wk, 'Jan 1, xxxx') always return 1. However, a lot of reports Week No is actually based on ISO 8601 standard, that is January 1 of any year can be Week 53/52 or Week 1. (http://www.merlyn.demon.co.uk/weekinfo.htm#IDC)

The Rule is as follows:
The first Week of a Year is Number 01, which is :-
*defined as being the week which contains the first Thursday of the Calendar year; which implies that it is also :-
*the first week which is mostly within the Calendar year,
*the week containing January 4th,
*the week starting with the Monday nearest to January 1st.

The conditions are mutually equivalent. See Markus Kuhn and R.H. van Gent, via datefmts.htm, for example.
The last Week of a Year, Number 52 or 53, therefore is :-
*the week which contains the last Thursday of the Calendar year;
*the last week which is mostly within the Calendar year;
*the week containing December 28th;
*the week ending with the Sunday nearest to December 31st.

Run the scripts below, and test

Declare @tDate Datetime
set @tDate =getdate()
select dbo.udf_DT_ISOWeekNum (@tDate)