HMRC allows charities to go back in time to collect Gift Aid or top-ups under the Gift Aid Small Donations Scheme.
An understanding of the windows available for doing this is integral to a solid grasp of Gift Aid.
Gift Aid
Gift Aid works around a charity's 'accounting year' - the annual accounting cycle. A charity's year end can occur at any time. Some groups of charities have mandatory year end dates: Anglican PCCs and Baptist churches always run with the calendar year; charitable trusts always run with the tax year, 06 April-05 April.
A charity is allowed to retrospectively claim on a donation up to four years after the end of the year in which the donation was made. As an example, a donation made to an Anglican PCC on 07 July 2012 is available for claiming up to 31 December 2016. The same donation made to a charitable trust would have a window up to 05 April 2017.
Gift Aid Small Donations Scheme (GASDS)
The GASDS scheme runs with the tax year, irrespective of the charity's accounting year, which may be different. The retrospective window for GASDS two years from the end of the year in which the donation was made. As an example, a donation made on 06 June 2013 can be included in a claim dated up to 05 April 2016, after which the window closes.