i wonder why when we value forward contract at t = t in derivatives we just make another forward contract maturity at the same time and compare with the two forward rate. but in forward foreign exchange rate, when we calculate mark-to-market, we need to construct an offsetting forward transaction, why don’t we just construct an new forward rate maturity at same time and with same long/short side. because the two different way to calculate mark-to-market will use different exchange rate, one for bid price and one for ask price, the result will be totally different.