MATLAB: Slow conversion from date in Matlab time to day of year

datenumdatestr

I have a vector of dates in Matlab time convention which I need to convert to day of the year. The following code is slow (9 seconds) even though it is using builtin Matlab functions. What do you recommend to speed it up?
timeGmt = datenum(1996,1,1):1/24:now; % Long vector of dates covering many years
timeGmt = timeGmt'; % Change from row vector to column vector for compatibility in next line
yearDay = timeGmt - ...
      datenum(str2num(datestr(timeGmt, 10)), 1, 1) ... % Subtract off first day of this year
      + 1; % Add a day so that first day of year is day 1

Best Answer

Try this variation using datevec instead of datestr:
vecGmt = datevec(floor(timeGmt)); % Set hms to 0's using floor
vecGmt(:,2:3) = 1; % Set month and day to Jan 1
yearDay = timeGmt - datenum(vecGmt) + 1;
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