Property held jointly by married couples or civil partners: Form 17 rule - when declaration takes effect: normal case
Income from property included in the declaration is split in the new way from the date of the declaration (which is the date the declaration was signed by the last spouse or civil partner to sign) provided the notice of declaration reaches the Inspector within 60 days of the date it was signed. See TSEM9874 about late and lost declarations.
For example, the husband signs on 10 June 2010, and the wife signs on 20 June 2010; the declaration applies to income that arises on and after 20 June 2010 provided the declaration reaches the Inspector within 60 days of 20 June 2010.
A declaration that is late is invalid (see TSEM9862); it has no effect at all. The couple must make a further declaration and send it to the Inspector within the 60 day time limit if they want income to be split on the basis of actual entitlement. Only income that arises after the date of the declaration is covered.