Plot time-of day histogram for array of datetimes

How do I create a histogram for an array of datetimes showing the counts of times of day from 00:00 to 24:00. I'd like the bin width to be variable between 1 to 60 minutes.

Answer 1

[dpb]

With datetimes, use the duration class for times of day..

[dormant]

Sorry, but I don't understand your answer.

I've only recently started using datetime and have no idea what a class is and how to use it.

[dpb]

So, we don't understand exactly what you have to start with, either. Show us a sample of your data set.

As a rough guess,

x=rescale(randn(1000,1),0,24*60*60-1);      % a set of seconds
d=duration(0,0,x,'format','hh:mm');         % turn into duration
histogram(d)

or

figure
tl=tiledlayout('flow');
nexttile(tl)
d=datetime(2025,1,1,0,0,x,'format','hh:mm');    % turn into datetime
histogram(d)
d=d-datetime(2025,1,1,'Format','hh:mm');        % convert to duration from beginning of day
nexttile(tl)
histogram(d)

gives back the first

Note that if the datetimes include more than one day, using them directly will bin by day and time, not just time; hence the use of a duration instead. The subtraction of the date from the array of values can be vectorized for all; one doesn't have to have a fixed date.

[dormant]

I've got an array of datetimes spanning a period of 5 years.

>> size( datetimeVTs )
ans =
           1        2544

>> datetimeVTs(1:5)
ans = 
  1×5 datetime array

   2020-01-01 07:22:06   2020-01-01 11:35:31   2020-01-02 04:44:33   2020-01-03 12:18:37   2020-01-04 07:19:49

I want to do a histogram that ignores the dates.

[dormant]

It is easier than I thought:

d=timeofday(datetimeVTs);
histogram(d);

[dormant]

Thanks for the help. you pointed me the right way.

[dpb]

No problem -- I should have recalled the timeofday function; subtracting the base datetime for each day is what it does. Seems like maybe when the datetime class was first introduced it wasn't yet there, maybe, is why I fixated on using the duration class; I don't recall precisely. I do remember there being an Answers Q? quite a long time ago about creating some specialized plots that the poster had initially done with datenum required recasting; maybe that recollection got me sidetracked.

Anyways, glad you looked in more depth on your own...