The highlighted countries (or some portion of them) observe Daylight Saving Time (or a better list of countries).
I chose to represent each country as a whole, even though certain parts of or localities in the United States, Canada, Mexico, Brazil, and Australia do not observe DST. Omitting these exceptional regions would have been hard to represent accurately on the map, and would probably have also made the question too easy! I also just noticed that I included Mongolia when I shouldn't have (they apparently stopped using DST in 2002).
Congratulations to kvarko, who skilfully used an impressive google search to help find the answer.
Daylight Saving Time ended in New Zealand this weekend. I've got two world clocks on my desktop, one representing GMT and one representing New Zealand time, and they are exactly 12 hours apart now. It's going to weird living in the southern hemisphere with the largest time offset possible from GMT!