Warming tray on Shabbat

Question

Right after I sent you my question, re:roast from the fridge to warming tray, I perused the board and found the question re:a hot plate on a timer. If I understand your answer to that and other questions on the board relating to warming liquids etc. The only way to warm a roast from the fridge on Shabbat is to put it on a warming tray while the tray is off, once the timer goes on it is too late to put the food on. Correct?