If your wallet.dat back up is outdated, do you lose your coins since the back-up? My last back up of wallet.dat was from the beginning of the year. My computer crashed recently and I don't have the most recent wallet.dat after receiving bitcoins. If I use my outdated wallet.dat, do I lose the new bitcoins I received?
If you made more than 100 transactions and individual receiving addresses combined, yes. 

Best way to find out is to try.