Sébastien,

Can you tell us what time.localtime(t) produces for t in (-2, -1, 0, 1)?  Apparently time.mktime() fails on values produced by time.localtime() and this sounds like a platform bug.  It is OK to restrict time_t to positive values, but in this case time.localtime(t) should reject t < 0.

If there is a Python issue here, it is likely to be in error detection in time.localtime().

Also, what is your timezone?