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

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K E
K E on 18 May 2015
Commented: Peter Perkins on 3 Jun 2015
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

Accepted Answer

James Tursa
James Tursa on 18 May 2015
Edited: James Tursa on 18 May 2015
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;

More Answers (1)

Peter Perkins
Peter Perkins on 18 May 2015
Edited: Walter Roberson on 19 May 2015
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.
  2 Comments
Peter Perkins
Peter Perkins on 3 Jun 2015
If you have R2014b or later, you have the day function. It is a method of the datetime class, which was introduced in R2014b.

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