Many ovens have a timer which allows you to set the start time, so you pop your item in and later in the day it will automatically start cooking it unmanned.
But all the oven cooking I can think of requires I preheat the oven before putting anything in it.
Is it that preheating is desirable rather than essential, or is it that the timer feature is designed for those few cases where preheating isn't required?
Preheating is mainly needed for breads and anything that has a short cooking time (< 15 mins or so). I don't bother preheating when I'm roasting something for a longer time. If you're setting a timer, I assume this is something that will take a while to cook, so I wouldn't worry about preheating.
I seldom preheat for the kind of cooking I do. Things like cookies and cakes probably need to be preheated. Anything where you cooking for long periods of time, especially if the food starts out cold, should be fine. The only thing to be careful of is how long the food will sit in the oven before it comes on.
Don't do this with your pizza but braising, roasting and casseroles seem to do OK without preheating. I'm OK with beef sitting in the oven without refrigeration but I have to brine chicken and pork to preserve it a little and it seems to help with taste and juiciness anyway.
I use the cold oven method for bread, which means I don't preheat and then move the bread in. To answer the question, the oven likely starts the timer when it turns on, not when it reaches the set temperature. You'd have to add that time to the bake time if baking at the set temperature is important to you. Preheat times vary. My oven takes about 9 minutes to reach 450, so I add 9 minutes to the bake time I want.
