Time is based of the rotation of the Earth. The Earth is rotating at a certain speed. One rotation of the Earth at a certain speed will always take the same duration (i.e. amount of time). Of course motion is relative and so is time, and while calling it an "illusion" is in a sense correct, the term seems to imply that it is false, deceptive or useless when in reality it's measurable, useful and practical. Weather that one Earth rotation "seemed" to take a long time or seemed to pass quickly is a result of your perception of time and not time itself. If anything is a misleading illusion it is your perceptions.