What is the earliest date that you can do Kapporos? Is it rosh hashanah?
(Note: I'm not asking for the best or most preferable time.)
yes and no
Mishna berrura 605.2
כי כל עשי"ת הוא זמן לכפרות [פמ"ג ע"ש].
All of the 10 days of teshuva is the time for kappros [pri megodim, see there]
from the maharal m'prag's book
נתיבות עולם - נתיב הבטחון - פרק א
it seems that anytime during the year when you lose a chicken (even when it is not shechted) it is a kaporo of your neshomo
P.S. but from other places i saw it seems to me that it is kaporos only when it shechted correctly (maybe i am wrong)