Thanks to a post in a Chinese Pokemon
forum, we have confirmation and further information of some forme patterns.
1) White/Black Kyurem, as well as the Resolute Forme Keldeo, are restricted to 12:00-1:00, both am and pm
2) Shaymin, Cherrim and apparently Darmanitan are day/night based. Not sure what is the exact time. Perhaps 8 to 8?
The following are also already on Serebii's pages:
1) Arceus cycling through the 18 formes daily (check Rumble's search function by type for the sequence)
2) Deoxys cycling through its 4 formes daily (N-A-D-S)
3) Genesect cycling through its 5 drives (normal-water-electric-fire-ice, as how the game sequences them)
4) Unown cycling from A to Z, then ! and ? from 1-28th each month, the rest of the month is a combo)
5) Giratina/Landorus/Thundurus/Tornadus - alternates daily
6) Deerling/Sawsbuck - alternates monthly (Jan-spring, Feb-summer and so on)
Other Pokemon with formes, including gender differences, are just random.
Still, there is no clue on the Fancy and Pokeball Vivillons though.
Also, for those who plan to tamper with the 3DS clock, there are drawbacks but allows you to get the cycling Pokemon much quicker (like Arceus).
Whenever you change your 3DS's clock or time, the Emperor will tell you that the Daily functions (20 diamonds/internet Mii recruitment...etc) will be suspended for 24-48 hours.
This apparently doesn't affect the daily/time-based Pokemon rotations though.
Say you entered an Arceus stage, and caught the type of the day. There should be a Mii who takes you back to your last played stage for 3 diamonds. Just set your clock to 23:59, wait til it hits 00:00, talk to the Mii and Arceus would have changed formes when you revisit. Repeat 18 times in a row and you get all Arceus formes. When you reset the date back to the correct one, it would not erase your Pokemon captured in the future
This applies to all other Pokemon. Be a bit careful when you do this for Deerling/Sawsbuck. Because apparently your visiting Miis will disappear with time, so if you change your date to June/July, you may not get that revisit Mii to help you out.
Coronis~