cohort: Events are recorded time-stamped observations for the
persons, such as drug exposures, conditions, procedures,
measurements and visits. All events have a start date and end
date, though some events may have a start date and end date
with the same value (such as procedures or measurements). The
event index date is set to be equal to the event start date.
restrict observation duration fromto
with continuous observation of at leastdays before and days after event index date