…depending on the languages used in their backend and the quality of their development efforts…

The balances are instantaneous balances, not ones that are being financed. So, if you use and pay off your card every month (as makes sense to me for points/cash back, ease of chargeback, and convenience), then your numerator won't be zero on most days. (For me, it's not $0 on any days, because of the timing of statements.)

You could theoretically pay off the card before the statement posts to keep the balance at zero. Usually not worth it, but if you want to boost your credit score before getting a mortgage, that trick usually works.