Historical crypto prices for the books: source, timestamp, evidence
Every crypto journal rests on a price. Where historical prices come from, why timestamps matter, and what auditors and tax authorities expect.
What this means for you: the value on a crypto journal line is only as good as the price behind it. You need a defensible source, a price that matches the moment of the transaction, and a record of both. Tax authorities and fair value standards ask for the same discipline, just in different words [1][2][4].
Where historical prices come from
There are three common sources. The first is the transaction itself. When you trade on an exchange, the IRS says the value is the US dollar amount the exchange recorded for that trade [1]. The second is market data from a specific venue, such as an exchange's trade or candle history. The third is an index or aggregator that blends quotes from many venues. For peer-to-peer and on-chain trades, the IRS accepts a value from an explorer or service that analyzes worldwide indices, taken at the date and time of the transaction [1].
When an asset has no published value at all, the IRS points to the fair market value of whatever was exchanged for it [1]. That is often the best evidence for a thin or new token.
Book fair value: the principal market
For US GAAP, ASU 2023-08 requires in-scope crypto assets to be remeasured at fair value, with changes in net income. The FASB concluded that ASC 820 already works for crypto, so the usual fair value rules apply [5].
Under ASC 820, fair value is measured in the principal market: the market with the greatest volume and level of activity for the asset that the entity can access. You use that market's price even if another venue quotes a better one. The market you normally transact in is presumed to be your principal market unless there is evidence otherwise [4].
Two practical points follow. Different entities can have different principal markets for the same token, because access differs. And an aggregator is not itself a market. Deloitte's guidance notes that aggregator data can help you find the principal market, but it cannot be designated as one [4]. If you use a blended index for valuation, document why it is a reasonable proxy for the price in your principal market.
Why the timestamp matters
Crypto markets trade around the clock and prices move within minutes. A daily closing price can be far from the price at the moment a trade happened. The IRS frames value at the date and time of the transaction [1]. In practice, that means matching each event to the finest reliable price interval you have, such as a minute candle that contains the trade time, rather than a single daily figure.
A day close is right for some things. Period-end remeasurement uses a price at your chosen cutoff, and that cutoff should be applied the same way every period. For a trade at 09:14 on a volatile day, a day close is a shortcut, and it should be flagged as one.
| Use | Price to look for | Watch out for |
|---|---|---|
| Exchange trade | The fill price the exchange recorded [1] | Fees netted into the fill |
| On-chain swap or payment | Market price at the block time [1] | Day-close prices on volatile days |
| Period-end fair value | Principal market price at your cutoff [4] | Changing cutoff times between periods |
| Thin or new token | Value of what was given up [1] | Single-venue prints with little volume |
UK: sterling values and a consistent method
HMRC needs gains and profits in pounds sterling. Where a trade has no sterling value, for example bitcoin for ether, you must establish an appropriate exchange rate. HMRC asks for reasonable care, a consistent methodology, and records of that methodology [2]. Businesses may keep a non-sterling functional currency, but the tax return is still prepared in sterling [3].
Stablecoins: the peg shortcut and its limits
Many teams value a recognized dollar stablecoin at 1.00 without looking up a price. That is a reasonable shortcut in normal markets, but it is a policy choice, not a rule, and it needs a depeg guard. In March 2023, after Circle disclosed 3.3 billion dollars of USDC reserves at Silicon Valley Bank, USDC traded as low as about 86 cents before recovering once depositors were protected and redemptions resumed [6]. A stablecoin trade booked at 1.00 that weekend would have overstated value.
Set a written depeg threshold, check the market price against it, and when it is breached, use the observed market price and flag the period for review.
Never use today's price for yesterday
Current spot is fine for a dashboard estimate. It is not evidence of what an asset was worth on a past date. If a historical price is missing, mark the item unpriced and resolve it, rather than filling the gap with today's quote. A silent substitution can distort gains, income and basis for years.
What to record with every price
- The source: exchange, venue data or index, and its methodology.
- The requested time (the transaction or cutoff) and the actual time of the price point returned.
- The interval: trade print, minute, hour or day.
- Any fallback used and why, with a review flag.
- The pricing policy version, so a later re-run can be reproduced.
The IRS expects records sufficient to support the positions on your return, including fair market value [1]. HMRC expects your valuation methodology to be kept [2]. Your auditor will ask the same questions for book fair value.
What to do
- Write a pricing policy: source order, principal market assessment, period-end cutoff time and depeg threshold.
- Price each event at the finest interval available that contains the transaction time.
- Store source, timestamps and interval alongside every valued line.
- Treat missing prices as exceptions to resolve, never as a reason to use spot.
- For UK reporting, convert to sterling with one documented, consistent method.
Sources
- Frequently asked questions on virtual currency transactions (Internal Revenue Service)
- CRYPTO23000 - Cryptoassets for individuals: valuation (HM Revenue & Customs)
- CRYPTO40100 - Cryptoassets for businesses: conversion to Sterling and accountancy (HM Revenue & Customs)
- 4.3 Principal Market (Roadmap: Digital Assets) (Deloitte DART)
- 4.2 Crypto Assets That Are Within the Scope of ASU 2023-08 (Deloitte DART)
- In the Shadow of Bank Run: Lessons from the Silicon Valley Bank Failure and Its Impact on Stablecoins (Board of Governors of the Federal Reserve System (FEDS Notes))
General information for finance teams, current as of publication. Not legal, tax or accounting advice.