I would wait until the morning of the 60th day before attempting to do online check in. The system can be glitchy for the first day or so which makes it better to wait a while.
There really isn't any reason to do online check in more than a week or so before check in date. They don't start assigning rooms until about five days out. And I like to take some time to think about what room requests I want to make. Once you make a room request in the online check in site you can't undo it.