How can monistat be good for? i lost the box for the expiration date?

Depends : Typical pharma dating will be for about 2 years from manufacture, but your date will depend on how ‘new’ the product is. How long was it in warehouse, pharmacy, your medicine chest, as well as if was stored in a cool dry (better) location vs a hot steamy one (worse). Net out, if well stored, fairly recently purchased and looks like new, why not use it? You will soon learn if it worked. Good luck