Crypto Facilities and the CME Group  have been calculating and publishing the Bitcoin Reference Rate (BRR) since November 2016. Such an official rate is a prerequisite of options trading in the traditional markets. The BRR is calculated based on the rates from the biggest exchanges: Bitstamp, GDAX, itBit, and Kraken. More concretely, it is calculated based on all Bitcoin vs. USD trades on the participating exchanges between 3 and 4 p.m. London time. To calculate the BRR, the hour between 3 and 4 is divided into 12 intervals of 5 minutes. For each interval, the volume-weighted median of the Bitcoin price is calculated (statistically, the median, in contrast to the average, prevents single outliers from distorting the price). The BRR is then the average of these 12 median values. More details about the calculation of the BRR can be found in the BRR whitepaper.
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The first timestamping scheme invented was the proof-of-work scheme. The most widely used proof-of-work schemes are based on SHA-256 and scrypt.[18] The latter now dominates over the world of cryptocurrencies, with at least 480 confirmed implementations.[37] Some other hashing algorithms that are used for proof-of-work include CryptoNight, Blake, SHA-3, and X11.
When a Fund purchases a put or call option on a futures contract, the Fund pays a premium for the right to sell or purchase the underlying futures contract for a specified price upon exercise at any time during the option period. By writing (selling) a put or call option on a futures contract, a Fund receives a premium in return for granting to the purchaser of the option the right to sell to or buy from the Fund the underlying futures contract for a specified price upon exercise at any time during the option period.
The Code generally imposes a 3.8% Medicare contribution tax on the net investment income of certain individuals, trusts, and estates to the extent their income exceeds certain threshold amounts. For these purposes, “net investment income” generally includes, among other things, (i) distributions paid by a Fund of ordinary dividends and capital gain dividends as described above, and (ii) any net gain from the sale, redemption or exchange of Fund shares. Shareholders are advised to consult their tax advisors regarding the possible implications of this additional tax on their investment in a Fund.
Investors in the ProShares Short Bitcoin Futures Strategy ETF should understand the consequences of the “short” strategy employed by the Fund, which is subject to, among others, compounding and market volatility risk. The Fund may adjust its short exposure as frequently as daily basis which entails obtaining additional short exposure as the Fund experiences gains, and reducing short exposure as the Fund experiences losses. As a result, the Fund’s performance may be more vulnerable to the effects of compounding than funds that do not seek to provide short investment exposure. During periods of high volatility, this risk may be exacerbated and the Fund may have losses as a result of such adjustments.
Slippage is when the actual price we execute at is different from what we expect. In this case, a perfect market would let us sell the bitcoin and settle the future at precisely the same time and price. But in practice, market imperfections and Bitcoin volatility could lead to the price moving between the two trades, wiping out profits or putting you in the red.

There's a lot of ins and outs which can get confusing in arbitrage trading. We won't go into the technicals of why futures contracts trade at a premium to spot price. You can read a full explanation here. If it's not of interest to you, all you need to know is that there's a tendency, the further out in time the futures contract expires, for the premium to spot to be higher and higher in nominal percentage terms.