According to the IRS’s Frequently Asked Questions on Virtual Currency Transactions, reporting cryptocurrency-related income in various scenario requires determining the market price at a given date and time.
For example (emphasis mine):
Q26. I received cryptocurrency in a peer-to-peer transaction […]. How do I determine the cryptocurrency’s fair market value at the time of receipt?
A26. If you receive cryptocurrency in a peer-to-peer transaction […], the fair market value of the cryptocurrency is determined as of the date and time the transaction is recorded on the distributed ledger, or would have been recorded on the ledger if it had been an on-chain transaction. The IRS will accept as evidence of fair market value the value as determined by a cryptocurrency or blockchain explorer that analyzes worldwide indices of a cryptocurrency and calculates the value of the cryptocurrency at an exact date and time. If you do not use an explorer value, you must establish that the value you used is an accurate representation of the cryptocurrency’s fair market value.
There is similar language for other scenarios.
Are there any publicly-available data sources for minute-by-minute historical Zcash pricing data? Which can be looked up programmatically, by precise time or by transaction ID?
It used to be common practice to use something like CoinMarketCap’s historical Zcash data for the day when the transaction occured, by averaging the day’s opening and closing prices. But this doesn’t seem to match the new “exact date and time” guidance.