The 23 days was for diseases, I believe.

[b]Rule XXVII: If you write, die of disease for the cause of death, but only write a specific time of death without the actual name of disease, the human will die from an adequate disease. But the Death Note can only operate within 23 days (in the human calendar). This is called the 23 day rule.[/b]

That thing about erasing doesn't make sense...

[b]Rule XI: The time and conditions of death can be changed, but once the victim's name has been written, the individual's death can never be avoided.[/b]
[b]Rule XLI: It is useless trying to erase names written in the Death Note with erasers or white-out.[/b]