Do you see, that you use "MM" for minutes and months? The format strings for the date format are case-sensitive.
b=datestr(y_new, 'dd/mm/YYYY');
d=datenum(b, 'dd/mm/YYYY');
The conversion over the string representation and back to a serial date number is indirect. See x2mdate. I've seen several workarounds, e.g.:
y = y + datenum(1900, 1, 1) - 2; % Or -1 ?!
c = fix(y); % date
d = rem(y, 1); % time
Do you have a modern Matlab version? Then the datetime objects are better than datestr.
