Converting a date string to day of year

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.

Answer 1

[Jan]

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));

[NewbieCA]

Thanks!

[Siyavuya Madlanga]

Thank you very much

[Peter Perkins]

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

Answer 2

[James Tursa]

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]

Thanks for this

Answer 3

[Ian]

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

[James Tursa]

Or, for a vectorized version:

day = v - datenum(d(:,1), 1,0);

[Peter Perkins]

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]

Wish I could vote on comments like StackOverflow - Peter's was useful for me.

Answer 4

[Toni]

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]

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!