"The ERC checks that my client has been getting are sometimes for more than the number days in the current year."

Because that isn't why the interest is computed and included.

https://www.ez-erc.com/received-your-erc-funds/

"the IRS also pays interest on the ERC refunds based on the periods for which the credit is being claimed"

https://www.barnesdennig.com/ertc-taxable-interest-income/

"Taxable interest

In addition to paying out the refund amount requested, the IRS adds interest to the refunds for the time value of money. Regardless of when the ERTC refunds are received, while the wage disallowance must be reflected on the income tax return for the year the credit is related to, interest is a different story.

In general, when taxpayers have their ERTC refunds issued by the IRS, they receive Notice CP210 in the mail indicating their refunds can be expected to be sent in the next few weeks. Page 2 of this notice generally states exactly what amount of the refund to be received is associated with interest. This interest amount is considered taxable income in the year received.

For example, if a taxpayer received a refund check in December 2022 for $103,000 for tax year 2020 ERTC and the notice indicates $3,000 of the amount being paid out is interest, the $3,000 must be picked up as interest income on the 2022 business income tax return. Consequently, if the refund check was received in January 2023, the $3,000 would be picked up as interest income on the 2023 business income tax return."

For the refund amount, the credit component of the refund is offset by amending the wage/benefit expense, so that meets the matching principle. For the interest earned component, that is not anticipated in the qualifying year against the credit. It is the result of filing after-the-fact.

The credit is from specific historic periods. Not from days only in the current year.