Instead of assuming a return rate, these tools replay what a real stock or fund actually did, day by day, with dividends reinvested, over the dates you pick.
A calculator projects forward from a rate you assume, the same smooth curve every time. These tools use a stock or fund's actual daily closing prices, adjusted for stock splits with dividends reinvested, so the result is what really happened over your dates, including the falls along the way. Past returns don't predict future results.