In order to use datetick on numeric values, the values need to be in MATLAB's numeric date format. That format represents time as number of days since Jan 1, of a mythical year 0. You need to convert 20120824 to that format in order to use datetick on numeric values, The easiest way would be
startDate = datenum(num2str(tday(1)),'YYYYMMDD');
endDate = datenum(num2str(tday(end)), 'YYYYMMDD');
The next problem you have is that you datetick() works on existing x values. When you used
plot(cumret)
implicit x values of 1:length(cumret) were used. You may have calculate a variable named "x" but you have not given any connection between that variable and the axis for your plot.
What you should do is calculate startDate and endDate first (with the conversion to date numbers), and then you should
x = linspace(startDate, endDate, length(cumret));
as that defines one date value for each entry in cumret. Then
plot(x, cumret);
datetick('x', 2);
