Hehe, I did this a month ago
5 billion years will be 365 days !
The most direct way is to calculate the difference between Earth's age and the apparition of Homo Sapiens. This would be 4.999 billion years. So divide it by 365 to get the number of days since the 1st of January.
Another way to do this is to start from the end.
You know the ratio date/days (365/5billion), so 1 million will be ... days from the end.