It starts from the date of expiry of your current leave.
Also there is you dont need to get extension for 2 year and some extra days.Read this :-
Read the page number 10 carefully i.e
In some cases, applicants may have been granted five years continuous leave, but due to delayed travel will not have spent five years continuously in the UK before their current leave expires. Caseworkers may count the period between entry clearance being granted and the date the applicant entered the UK towards the five years, provided this period was not longer than three months.
http://www.ukba.homeoffice.gov.uk/sitec ... iew=Binary