Here are a few lines of "zdump" output for America/Sao_Paulo: America/Sao_Paulo Sun Oct 19 03:00:00 2003 UTC = Sun Oct 19 01:00:00 2003 BRST isdst=1 America/Sao_Paulo Sun Feb 15 01:59:59 2004 UTC = Sat Feb 14 23:59:59 2004 BRST isdst=1 America/Sao_Paulo Sun Feb 15 02:00:00 2004 UTC = Sat Feb 14 23:00:00 2004 BRT isdst=0 America/Sao_Paulo Sun Oct 17 02:59:59 2004 UTC = Sat Oct 16 23:59:59 2004 BRT isdst=0 The important thing to note is at midnight on February 14 clocks are set back an hour. This means that there are two instants when a well-maintained Sao Paulo wall clock would read 11:00:00 PM on February 14 (as well as two instants when it would 11:00:01, 11:00:02, and so on through 11:59:59). The "my_date.c" program feeds year-mon-mday-hour-min-sec into mktime; the values being fed in lie in the "fall back" hour above. Mktime is supposed to return an instant when a wall clock shows the time that's been input to it; here, since there are two such instants (and since "tm_isdst" has been set to -1), mktime is allowed to pick either instant to return. It's luck of the draw that determines which instant mktime returns. --ado