In the original example, the behaviour would arise from the strptime() call -- on most systems this ends up being a call to the strptime function in your local C library.

As it so happens, you were using %Y in your call which means the year, including the century (i.e. 4 digit year for us mortals).

On the other hand, had you used %y, it is defined thusly:

%y
    is the year within century. When a century is not otherwise specif
+ied, values in the range 69-99 refer to years in the twentieth centur
+y (1969 to 1999 inclusive); values in the range 00-68 refer to years 
+in the twenty-first century (2000 to 2068 inclusive). Leading zeros a
+re permitted but not required.
[download]

That's the source of the difference of opinion.

Cheers,
Matt