Ah, I realize I didn't really address your original question. Some sources sample returns each trading day, and use either the actual number of trading days that year, or a fixed value 252 or 250 as an approximation. This has a bias because every ~5th trading day represents 3x as much economic time (due to weekends) but I believe most sources assume the bias is small enough to ignore. A related topic in bonds is the "day count convention" -- https://en.wikipedia.org/wiki/Day_count_convention -- where they actually have to carefully spell out the details. If you figure this stuff out and feel so inclined it could make a good blog post or twitter thread.