Maximum allowed expiry date value is 1 day off

I've set both default and maximum expiry dates to +60 day. The user can not create the node if a date +60 days is set. The penultimate day is permitted however (i.e. 59 days later). If it makes any difference I was doing this at 2am.

e.g. if the maximum date allows values up to and including July 4. If the user selects July 4 as the expiry date then it will be rejected. July 3 is accepted. This is not expected as the JQuery UI allows July 4 to be selected.