Why slow conversion from date in Matlab time to day of year?

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

Answer 1

[James Tursa]

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;

[K E]

Awesome! 0.15s instead of 8.8s on my machine!

[James Tursa]

Try the edited version using floor instead of setting the hms to 0's directly.

[K E]

0.04s. very nice.

Answer 2

[Peter Perkins]

In R2014b or later, use datetime. On a not particularly fast Win7 machine:

>> timeGmt = '1-Jan-1996':hours(1):datetime('now');
>> tic, dayOfYear = day(timeGmt,'dayofyear'); toc
Elapsed time is 0.041145 seconds.

[K E]

I don't seem to have the function day.m but it may be part of the Financial toolbox .

[Peter Perkins]

If you have R2014b or later, you have the day function. It is a method of the datetime class, which was introduced in R2014b.