So realistically, let's say this project gets all approvals within the year and construction begins next spring (2019) on the stadium. We talking about playing in a new stadium by 2021?
Unfortunately it won't get approvals within the year even if they announce it tomorrow - the ULURP (i.e. rezoning and special permit) process takes about a year in itself. So unless we get an announcement and official filings/certification/etc. in the next few weeks we're still talking end of the 2019 season at the very earliest before we see a shovel in the ground.
EDIT: Also what Ulrich said - depends on how much remediation is needed too.