The proposal process does not have a specific deadline, per se, but instead is part of a continuum leading to an approved proposal, followed by thesis course registration, thesis completion, and graduation.
There are three graduation timelines each year, shown here, corresponding to the three annual graduation events that happen in March, May, and November. Students typically pick a target graduation date and work backwards to consider which timing milestones they will need to adhere to make their graduation goal.
In order to graduate in May 2019, your proposal should be
approved during May 2018
and you should then be
registered for the thesis course by June 1, 2018.