I was going to say the same. The earth is 4.6 billion years old whereas humans have been around for about 200,000 years, depending on how you define humans.
200,000 divided by 4.6 billion = 0.00004347826087
There are 84,600 seconds in a day, so that means humans have been around for 3.76 seconds, if the entire history of the earth was 24 hours.
My result is different than amiable atheist because he uses 500,000 years rather than 200,000 years as the history of man.
If the history of the earth was 24 hours, then maybe mammals first appeared around about half an hour before midnight. That might be where the misunderstanding comes from.
edit: I see amiable atheist has changed his calculation from 500,000 to 300,000 for the history of man, so our results are now closer together.