Depends.
The definition of a 'year' can represent the orbital period of a planetary body, an approximation of the number of days in Earth's orbital period, or 31,557,600 seconds in a Julian year.
And let's not forget academic years, fiscal years, etc.
Are you starting from the beginning of the first year to the end of the last year?
Are we counting days, so that we have to include leap years?
Are we using one calendar over another?
If you are not going to be any more precise in your question then you can pick either of the two years surrounding the time that equally divides your total.