Just to point this out, The answer on these forums is always 11:59 p.m., But when you ask member services they will tell you the end of their business day. There were multiple people, myself included, who did not get to bank their points at the end of July this year because they cut people off at the end of their business day. That evening after work it told me I had no points available to bank on the website. When I called I was told there was nothing they could do.
I don't know if this is addressed in our contract somewhere, but it might be safer to tell people that the rule is end of business day and you may get lucky and be able to do it on the site.