A research calendar turns a multi-year project into weekly commitments, which is what makes it survivable. The failure mode in doctoral work is rarely a single missed deadline; it is months of drift with no visible marker until a milestone arrives and nothing is ready.
Schedule writing as fixed appointments rather than something done when time allows, because time never allows. Short regular sessions produce more finished text over a year than occasional long ones, and they keep the argument live in your mind between sessions.
Anchor the calendar to external checkpoints you do not control — supervision meetings, committee reviews, conference abstract deadlines, ethics submission dates — and work the internal tasks around them. Review it weekly, move what slipped rather than silently abandoning it, and keep it honest: a calendar that has stopped matching reality is worse than none.
Mark the seasonal constraints of academic life as well. University vacations, semester examinations and festival periods make respondents, especially students and faculty, much harder to reach, and they slow down committee meetings and supervisor responses. Planning data collection around those windows, and entering the six-monthly progress report dates your university requires, prevents the most common scheduling surprises.