When you have multiple purchases of a security, you can enter various as the purchase date.
If you don't know how many of the shares were purchased before 2011 and how many in 2011 or later, you could call them all Sales Category E (or Sales Category B, if some were short term) and list either the actual date sold, or, if there were multiple sale dates in 2016, use 12/31/2016. You will need to segregate the short-term from the long-term sales.
The general rule is FIFO - first in, first out. Your oldest shares were sold first.
