Your code confuses a date vector (which is a simple double matrix of size Nx6 or Nx3) with a datetime object (which has the property obj.Year). If you want to use that property then you will need to use a datetime object. If you want to use datvec, then you do not need a loop as it accepts a cell array of char vectors directly. So probably something like this will work (it depends on how the data is arranged in data):
mat = datevec(data(:,8),'yyyy-mm-dd HH:MM:SS');
year = mat(:,1);
