You can do this easily with datetime:
x=1.511092230713565e+05;
% The date part is the part before the decimal, so use floor
d = datetime(string(floor(x)),'InputFormat','yyMMdd')
% The time part is fractions of a day, remove the date part and call days
% to get a duration
t = days(x-floor(x))
% The result is the date part (which will be midnight) plus the fraction of
% a day
dt = d+t
% If you really must use datenum (though this is not recommended, datetime is so much better!)
datenum(dt)
