If someone pays me for a service in bitcoins (e.g. GPU rental), what dollar amount do I report for income? The dollar value of the bitcoin at the time I was paid? The dollar value on 12/31/2017 at 23:59?
If you later sold it for more, you owe tax on the gains.
then when you sell the btc for usd, you pay the capital gains tax (probably 15%, it depends) on the difference from the price it was accepted at, and the price it was sold at