An island changes country every six months.
Pheasant Island in the border river Bidasoa covers 0.68 hectares, about 200 by 40 metres. Since the 1659 Treaty of the Pyrenees it has been a condominium: Spain administers it from 1 February to 31 July, France from 1 August to 31 January. Nobody is normally allowed to set foot on it; naval staff from both countries inspect it regularly.
Useful: The world's smallest condominium shows sovereignty can also be a calendar.
Source: en.wikipedia.org