We usually receive ample warning.
Though it is still 2018, the 2019 Season started back on July 9, 2018 and will continue until sometime in July 2019. The 2019 Standard Format went into effect August 28, 2018. The reason the years seem "off" is that both are named for the calendar year when the World Championship for that Season takes place
and the powers-that-be want who is going to Worlds via an invitation settled well before the event. Invitations are earned via Championship points, and Championship points are earned through placing high enough in Championship Series events (see
here). The 2019 Standard Format went into effect on August 2018, 2018
but was officially announced on May 15, 2018, almost exactly 3.5 months ahead.
So... as stated, we're in the 2019 Season and the 2019 Standard Format. The 2020 Season will technically start well before the 2019 World Championship and the adoption of the 2020 Standard Format. The 2020 Standard Format will probably go into effect either late in August or early in September of 2019, with the announcement sometime in May of 2019. The 2019 World Championship will most likely occur in August of 2019.