Answer: when the diary becomes a blog, and this is what’s happening with the diary of George Orwell. According to this Telegraph article (George Orwell diaries to be published as blog),
The first entry will be posted on August 9, exactly 70 years to the day since the author of Animal Farm and Nineteen Eighty-Four wrote it. ((Martin. Nicole. “George Orwell diaries to be published as blog,” Telegraph 30 Jul 2008. 3 Aug 2008 <http://www.telegraph.co.uk/news/uknews/2469672/George-Orwell-diaries-to-be-published-as-blog.html>))
Presumably, the blog posts will be submitted in real time plus 70 years going through to his last entry in 1942. They will cover the start of World War II and include his travels to Morocco following injuries fighting during the Spanish Civil War in Catalonia.