at attempts to parse the date and time given and runs it through mktime() with "Daylight Saving Time" set to -1 (not available, auto-detect).
at source code:
tm1.tm_isdst = -1;
t = mktime (&tm1);
man 3 mktime:
So mktime() decides whether DST is or is not in effect; and it seems to prefer the later date out of two possible choices (although it's not clear from documentation how that decision is made).
