There is a systemic error in the Python documentation on time (such as 
<http://docs.python.org/library/time.html>). The term UTC is used 
incorrectly in the documentation where the term UT1 should be used.

The difference is that UTC includes leap seconds, whereas UT1 does not (see <http://en.wikipedia.org/wiki/Coordinated_Universal_Time>). The 
conversion routines ignore leap seconds, e.g., (time.mktime((2000,1,1,12,0,0,0,0,0))-
time.mktime((1990,1,1,12,0,0,0,0,0))) returns an exact multiple of 
24*60*60 and does not include all the leap seconds added between those 
dates. Using UT1 is more sensible that using UTC, since with UTC it is 
impossible to know when future leap seconds will be added, and hence 
impossible to convert future times to a number of seconds past the 
epoch.