OS: Windows 7 Professional (64-bit)
Hardware: Intel

datetime.strptime returns an incorrect value when calculating a date using a week number format, such as "%Y-%W-%w" (Year-Week-Weekday). The value returned for weekday '0' of a given week is consistently 7 days greater than it should be. The following code illustrates:

from datetime import datetime
for i in range(0,53):
    if i == 0:
        yr=input("Enter a valid year: ")
        print("Wk#\tBeginning of week\tEnd of week")
    BegWk = datetime.strptime((yr + "-" + str(i) + "-0"),"%Y-%W-%w")
    EndWk = datetime.strptime((yr + "-" + str(i) + "-6"),"%Y-%W-%w")
    print(str(i) + "\t" + str(BegWk) + "\t" +str(EndWk))

Here is a clip (7 lines) of the output from the code above:
Enter a valid year: 2013
Wk#	Beginning of week	End of week
0	2013-01-06 00:00:00	2013-01-05 00:00:00
1	2013-01-13 00:00:00	2013-01-12 00:00:00
2	2013-01-20 00:00:00	2013-01-19 00:00:00
3	2013-01-27 00:00:00	2013-01-26 00:00:00
4	2013-02-03 00:00:00	2013-02-02 00:00:00
5	2013-02-10 00:00:00	2013-02-09 00:00:00
6	2013-02-17 00:00:00	2013-02-16 00:00:00

The value returned for the first column of each week is exactly 7 days higher than the correct result.

from the man page for strptime:

       %w     The weekday number (0-6) with Sunday = 0.

       %W     The week number with Monday the first day of  the  week  (0-53).
              The first Monday of January is the first day of week 1.

Python's documentation is just a tiny bit clearer about %W:

       %W     Week number of the year (Monday as the first day of the week) as a decimal number. All days in a new year preceding the first Monday are considered to be in week 0.

So, the result is correct, albeit very unintuitive (the week day numbers for the purposes of %W are the sequence 1-2-3-4-5-6-0).  You will note that if you call strftime with the same format string, you will get your input strings out as the output.

Call this a design bug in posix that python has inherited.  You will get the exact same behavior if you write a C program that calls strptime/strftime.