30 April 1966: broadcast of "A Holiday for the Doctor", first episode of the story we now call The Gunfighters. The Doctor, Steven and Dodo arrive in Tombstone, Arizona in 1881 in search of a dentist.
30 April 2005: broadcast of Dalek. The Doctor and Rose encounter a captive Dalek in 2012 America.
30 April 2011: broadcast of Day of the Moon.
ii) date almost specified in canon
30 April, some time in the 1970s: most of the events of The Dæmons (1971).
Two more months to go of this project. I'm very grateful to David Haddock for suggesting a way of making it a permanent installation with Google Calendar; would be glad of any further offers of help in making it happen!