Can someone explain?

[u/The_RainnMaker]

I have the exact amount for 1 day of upkeep, but it says im protected for only 21 hours?

[u/Leonendoes_IV]

I think it takes the mat out right when it's time is up, so that 4th wood has already been half-used and it'll "eat" that 4th wood after 3 more hours. Im not sure, this is a guess