These dates were set in 2015. The debates time & place are determined by Commission on Presidential Debates.
course, Mike Mcurry is a Co-Chair.
http://www.debates.org/index.php?mact=News,cntnt01,detail,0&cntnt01articleid=57&cntnt01origid=15&cntnt01detailtemplate=newspage&cntnt01returnid=80
These dates were set in 2015.
...
They already knew Hillary was going to be the nominee in 2015.