bring it indoors now. although formaldehyde freezes at minus -97c cold is its nemesis and will make it form a white precipitate in the bottle know as paraformaldehyde which is toxic
so keep it in the dark but at room temp and it should last ages, check your stock buy pouring some into a clear sided glass and check for white bits floating in it or a thick white fog, if you see that dont use it dump it it should look like a glass of plain water from the tap..
but if you have a lot in stock and its showing these signs , contact me I have a work around to return it to usable formalin
dunc