"This will make your entry show up in January 1970 rather than April 2014 on your Recent Entries page, but it will still appear on your subscribers' Reading Pages, unless you select 'don't show on reading page'. Additionally, this entry will be crossposted with 'don't show on reading page' checked, and you will not be able to uncheck that on the other site(s). Post anyway?" would work for the "I am posting this from the paaaast!" side of things, but wouldn't help if you accidentally posted from the future and didn't realize until your next entry failed to crosspost.