Sherlock announcement to be made at 2.21pm?

Sherlock fans are bracing themselves for a critical announcement about the show today. Speculation over a series of ambiguous posts by the BBC One Twitter account reached fever pitch yesterday when a picture of the show's notorious villain Jim Moriarity appeared alongside the hashtag #221back.

"Did you miss me?" #221back. #Sherlockhttp://t.co/sJXE1VeFaG

— Sherlock - #221back (@BBCOne) July 1, 2014

But what does #221 mean? Will the announcement be made at 2.21pm? The phrase "did you miss me" also immediately sparked speculation the character, who killed himself at the end of the second series, would make a return in the fourth series. Or it could be to confirm reports of a Christmas special in 2015.