This calculator assumes 4 billion guesses per second, which is a....rather high number.
The thing that these type of calculators don't account for is that any decent software is going to lock you out for several minutes at least, after just a few incorrect attempts.
So if we assume a 15 minute lockout after 5 incorrect guesses, that would be 0.0055 guesses per second. So take whatever time figure you were given and multiply it by about 720,000,000,000.