This was originally reported on StackOverflow (53829118) and I believe the poster has found a genuine issue. He reported a problem converting from Python 2.3 to Python 2.7 in which strptime() produced a different result for %U in the two versions. For lack of an old enough copy of Python, I can not reproduce the Python 2.3 result, which he reports as follows:

Python 2.3.4
------------
>>> dw='51 0 18' # 51 week number, 0 for Sunday and 18 for year 2018
>>> date=time.strptime(dw,"%U %w %y")
>>> print date 
(2018, 12, 16, 0, 0, 0, 6, 350, -1) # 2018 12 16
[Remark: This output looks like Python 2.1 to me, but the issue is not the datatype of the result but the value of the result.]

Python 2.7.5
------------
>>> dw='51 0 18' # 51 week number, 0 for Sunday and 18 for year 2018
>>> date=time.strptime(dw,"%U %w %y")
>>> print date
time.struct_time(tm_year=2018, tm_mon=12, tm_mday=23, tm_hour=0, tm_min=0, tm_sec=0, tm_wday=6, tm_yday=357, tm_isdst=-1)

The point here is that the day of the month has shifted from 16 December to 23 December, and I believe that 16 December is correct.

In ISO week numbers, week 51 in 2018 runs from Monday 17 to Sunday 23 December. So the Python 2.7.5 result is correct for ISO week numbers. Only, ISO week numbers are provided by directive %W. 

%U is supposed to work with the week numbering system common (as I understand it) in North America, where (according to Wikipedia) week 1 begins on a Sunday, and contains both 1 January and the first Saturday of the year. While I am not familiar with that system, Excel 2016 is, and it reports 

=WEEKNUM(DATE(2018,12,16))  as 51
=ISOWEEKNUM(DATE(2018,12,16))  as 50 

But if I do the following in Python (2.6, 2.7 or 3.5) I get the week numbers reported as the same:

>>> dw='51 0 18' # 51 week number, 0 for Sunday and 18 for year 2018
>>> time.strptime(dw,"%U %w %y") == time.strptime(dw,"%W %w %y")
True
[Should be False]

So directives %U and %W are producing the equal results for this date, and further checking shows that the same unexpected equality appears for all Sundays in 2018. And I get the same unexpected equality for the Sunday of the 51st week of years 2063, 2057, 2052, 2046, 2035, 2027, 2007, 2001. It looks to recur when 1 January of a given year is a Monday. 

Now, it may be going too far to say that Excel is right and the Python standard library is wrong. It is clear that the algorithms are just systematically different. On the other hand, it appears that Python 2.3 did it the way that Excel does, and that raises the question of why Python does it differently now. 

A bit of searching reveals that people who complain that Excel's WEEKNUM function is wrong are generally just unaware that there are competing systems. So this difference is not in the same category as Excel's numbering of days before 1 March 1900.