How do they work/get the info?

Is it only possible to forecast the very next one?

Good question as several sites all use a site that at first glance is not very impressive. But they somehow get access to Blizzard data. The site displays not just TZs but also dclone status for all regions. The best answer I was able to find is that the one person behind this (his website provides API so other sites can display status). I found mention that he "sniffs" data from D2R servers and is somehow able to predict the TZ cycle.

As far as well know, the guy behind this is a Blizzard employee or a friend of a friend who has this access.

magic 8 ball

So nobody really knows for sure. And if they can sniff, why not the next 2-3 Zones … hmmm… ^^

blizzard sends the info to the servers beforehand so the transition from one tz to the next is smooth. That's why they can tell which tz comes after the current one.

i'm considering a new career path as a packet sniffer. any advice from pros ?

"the one" has an uncle working in blizzard

umm...sniff the packets, not the butts