Time decay can occur both during trading hours and overnight.
You probably know this, but keep in mind that Black-Scholes (and other options pricing models) are just models.There is no way to perfectly, or even near perfectly, calculate time decay with precision, because the only thing that actually affects an option's price is the order flow. For example, if buyers suddenly decide they're willing to pay more for an option near expiration, the option could stay the same price (or even increase) despite what should be strong time decay. This can happen even if the stock price hasn't moved. The options pricing model would explain this phenomenon by increasing the option's IV (i.e., the IV increase offsets the theta).
Not sure if this fully answered your question, but hope it helps somewhat.