# Converting a date string to day of year

145 views (last 30 days)
NewbieCA on 21 Feb 2011
Commented: Peter Perkins on 2 Mar 2022
Hi there I have a simple question that I can't figure out an answer to. I have a column of dates in mm/dd/yyyy format and I need to convert the dates to a Day of Year (1 to 365/366) and then export a file that has the year in column A and the Day of Year in Column B. The data set starts on Jan 1st 1957 so obviously includes leap years. Any advice would be greatly appreciated. Thanks.

Jan on 22 Feb 2011
Assuming that your input data is a cell string:
DC = {'01/02/2010'; '02/02/2010'};
DV = datevec(DC); % [N x 6] array
DV = DV(:, 1:3); % [N x 3] array, no time
DV2 = DV;
DV2(:, 2:3) = 0; % [N x 3], day before 01.Jan
Result = cat(2, DV(:, 1), datenum(DV) - datenum(DV2));
##### 3 CommentsShow 1 older commentHide 1 older comment
Siyavuya Madlanga on 6 Dec 2017
Thank you very much
Peter Perkins on 19 Dec 2017
Since R2014b, using datetime is a much better choice:
>> d = datetime('now')
d =
datetime
18-Dec-2017 22:20:02
>> doy = day(d,'dayofyear')
doy =
352

James Tursa on 22 Feb 2011
Here is an outline of code to get Day Of Year:
>> d = '12/25/1957'
d =
12/25/1957
>> v = datevec(d)
v =
1957 12 25 0 0 0
>> v0 = v
v0 =
1957 12 25 0 0 0
>> v0(:,2:3) = 1
v0 =
1957 1 1 0 0 0
>> datenum(v) - datenum(v0) + 1
ans =
359
NewbieCA on 22 Feb 2011
Thanks for this

Ian on 27 Jul 2015
Edited: Ian on 27 Jul 2015
As Tursa's example implies, matlab datenum's are numeric timestamps (of type double) that can be subtracted (to get a relative offset) or added to. They are simply time elapsed (in days, & can be fractional) since Jan. 0, year 0000. (See 'doc datenum').
A slightly more complete/compact version of Tursa's answer:
d = datevec('12/25/1957');
v = datenum(d); % v now == [1957 12 25];
day = v - datenum(d(1), 1,0); % datenum(yr,1,0) == datenum(yr-1,12,31)
Datenum( ... ) will operate on arrays of dates or datevecs, so a little creative programming can extract the day-of-year for an array of dates given as strings.
Matlab's double's are (currently) ieee 754 format, so the precision is about 1e-10 days, or ballpark 10 usecs, as the following example shows:
i
>> d1=datenum(now);
>> d2=[d1+1e-10, d1+5e-11];
>> d2==d1
ans =
0 1
##### 3 CommentsShow 1 older commentHide 1 older comment
Peter Perkins on 27 Jul 2015
Or, in R2014b or later,
>> d = datetime({'12/25/1957' '12/25/1960'})
d =
25-Dec-1957 25-Dec-1960
>> day(d,'dayofyear')
ans =
359 360
>> cellstr(d,'yyyy-D')
ans =
'1957-359' '1960-360'
K E on 19 May 2016
Edited: K E on 19 May 2016
Wish I could vote on comments like StackOverflow - Peter's was useful for me.

Toni on 24 Feb 2022
If you need convert a MATLAB datetime to a string with day-of-year,
The datestr function does not provide a format code for day-of-year. For example:
dt = datetime(2022,2,24)
datestr(dt,'yyyy-DDD')
results in: 2022-Thu
However, the string function can be used:
string(dt,'yyyy-DDD')
results in: 2022-055
Peter Perkins on 2 Mar 2022
Right, and in fact datestr on a datetime is just for backwards compatibility. Even when the input is a datetime, the format is interpreted as an "old-style" datestr format. Best to not use datestr any more!