I am not trying to be rude.Sorry if it sounds that way.If you do it manually it creates a likely chance of you forgetting to turn it on or off.(After all I am assuming you smoke pot)
if you just plain don't want to buy a timer, it wont hurt if it's off by a minute or so. But you better make sure you're home everytime you need to change the lights, and you have to remember to do it as well.