For tax purpose I want to calculate exchange rates for a given day in the past. I've done a look and cannot find a calculator that includes dates and is intended as and end consumer rate.
It depends on the country.
For US, the IRS publishes annual exchange rates and a list of daily/monthly sources: https://www.irs.gov/individuals/international-taxpayers/foreign-currency-and-currency-exchange-rates
For Canada, the CRA references the daily/monthly/anunual exchange rates published by the Central Bank: https://www.canada.ca/en/revenue-agency/services/tax/individuals/frequently-asked-questions-individuals/what-average-exchange-rates.html
For Australia, The Australian tax offices(ATO) publishes daily / monthly / annual exchange rates: https://www.ato.gov.au/Rates/Foreign-exchange-rates/
My go-to site is O&A (oanda.com).
They have a free tool on their site that will give you historical exchange rates between almost any pair of currencies, with bid/ask/avg and high/low /avg figures by day back to 1990.