(FACT-128) Remove use of non-standard 'daylight' variable

The daylight variable is used, but it seems to be non-standard. At least
it doesn't match what's described here:

http://pubs.opengroup.org/onlinepubs/7908799/xsh/time.h.html

which states "The value of tm_isdst is positive if Daylight Saving Time
is in effect, 0 if Daylight Saving Time is not in effect, and negative
if the information is not available."
Seems like a better option.