Let's say I want to bake a cake specifically on Monday night, so it can be fresh for Tuesday (no, I don't want to do it on Sunday). But I know that on Monday I am going to be extremely busy. So I thought, why don't I measure all the dry ingredients (flour, sugar, cocoa, baking powder, spices like cinnamon, and baking soda/salt), beforehand, a few days before, and leave them already mixed in a container/tupperware? In this way I already measured, cleaned everything, and I save up that part of the time.
Is it bad for these common cake ingredients to be together for some time? This is in a few days, but let's say you want to prepare for a day you are lazy but fancy baking a nice cake, but quicker. It would be like "homemade instant cake mix" but needing to add butter, eggs and milk, basically.