Question

I have a dataset for which I have extracted the date at which an event occurred. The date is in the format of MMDDYY although MatLab does not show leading zeros so often it’s MDDYY.

Is there a method to find the mean or median (I could use either) date? median works fine when there is an odd number of days but for even numbers I believe it is averaging the two middle ones which doesn’t produce sensible values. I’ve been trying to convert the dates to a MatLab format with regexp and put it back together but I haven’t gotten it to work. Thanks

dates=[32381 41081  40581  32381  32981 41081   40981  40581];

Answer 1

You can use datenum to convert dates to a serial date number (1 at 01/01/0000, 2 at 02/01/0000, 367 at 01/01/0001, etc.):

strDate='27112011';
numDate = datenum(strDate,'ddmmyyyy')

Any arithmetic operation can then be performed on these date numbers, like taking a mean or median:

mean(numDates)
median(numDates)

The only problem here, is that you don’t have your dates in a string type, but as numbers. Luckily datenum also accepts numeric input, but you’ll have to give the day, month and year separated in a vector:

numDate = datenum([year month day])

or as rows in a matrix if you have multiple timestamps.

So for your specified example data:

dates=[32381 41081  40581  32381  32981 41081   40981  40581];
years  = mod(dates,100);
dates  = (dates-years)./100;
days   = mod(dates,100);
months = (dates-days)./100;
years = years + 1900; % set the years to the 20th century

numDates = datenum([years(:) months(:) days(:)]);
fprintf('The mean date is %s\n', datestr(mean(numDates)));
fprintf('The median date is %s\n', datestr(median(numDates)));

In this example I converted the resulting mean and median back to a readable date format using datestr, which takes the serial date number as input.