Create a function to find elapsed time with string of date and times through samples?

Hi all.

I have a string array of dates in the form: '2016/10/05 1:06:58 PM' or '2016/10/06 3:01:46 AM'.

I am trying to write a function to find the time elapsed between each event be individual sample where each column (of 27) is an individual sample.

I have this:

function [timeBetween] = GetTimeBetweenFills(dateString)

      timeBetween = zeros(length(dateString),1);

      timeBetween(1) = 0;

      for i=2:length(dateString)-1

          time1 = datevec(dateString(i),'yyyy/mm/dd HH:MM:SS AM');

          time2 = datevec(dateString(i+1),'yyyy/mm/dd HH:MM:SS AM');

          timeBetween(i) = etime(time2,time1);

      end

end

And I realized that AM and PM changes through the sample, which I don't know how to handle, and I'm having issues running it through this loop to go through each sample:

S = size(SiteDates);

strArray = table2array(SiteDates);

timeBetween = zeros(S(1),S(2));

for j = 1:27

      SiDa = strArray(:,j);   

      timeBetween(:,j) = GetTimeBetweenFills(SiDa);  

end

SiteDates is a table and I tried to convert it to an array and have a string array now but MATLAB doesn't like what I've done.

I get the error:

Error using dtstr2dtvecmx
Failed to convert from text to date number.

Error in datevec (line 122)
            y = dtstr2dtvecmx(t,icu_dtformat);

Error in GetTimeBetweenFills (line 9)
        time1 = datevec(dateString(i),'yyyy/mm/dd HH:MM:SS AM');

Any help would be wonderful. This is my first time working with time in MATLAB.

Thanks!

Answer 1

[Sean de Wolski]

Please please please use datetimes. Then you can just subtract them (and they account for timezones, dst, leap seconds if you want, etc.)...

datetime('now')-datetime("2018-Jul-4 15:34")

For more info:

doc datetime

[Erin Winkler]

I can use that but how do I account for PM and AM?

[Erin Winkler]

Or does it know how to handle the difference?

[Steven Lord]

s = '2016/10/05 1:06:58 PM';
f = 'yyyy/MM/dd h:mm:ss a';
d = datetime(s, 'InputFormat', f, 'Format', f)

The 'a' identifier in the format f matches the day period. See the tables in the documentation for the Format property of datetime arrays for a list of the identifiers you can use in a format. I used the format both as the InputFormat (to match what is in your s array) and as the Format (to control how d gets displayed) but they don't necessarily have to be the same.

[Erin Winkler]

This works so beautifully. Thank you so much. The formatting is much easier to work with too. I really appreciate your help, thank you!

[Erin Winkler]

Actually, now I'm getting this error:

The following error occurred converting from duration to double:
Undefined function 'double' for input arguments of type 'duration'. To convert from durations to numeric, use the SECONDS, MINUTES, HOURS, DAYS, or YEARS functions.

[Erin Winkler]

So I have a cell array of date times formatted as above and sorted.

So, to loop through it all, I have:

      timeBetween = zeros(length(dateString),1);

      timeBetween(1) = 0;

      f = 'yyyy/MM/dd h:mm:ss a';

      for i = 2:length(dateString)-1

          t1 = char(dateString(i));

          t2 = char(dateString(i+1));

          d1 = datetime(t1,'InputFormat',f,'Format',f);

          d2 = datetime(t2,'InputFormat',f,'Format',f);

          timeBetween(i) = d2-d1;

end

But I'm still getting the same error. I thought converting the cells to char would help but the error is the same and it's alling specifically to the line

timeBetween(i) = d2 - d1;

But I don't understand why.

[Erin Winkler]

Wait, I think I understand. Duration is a type so I can use seconds() to convert and store it and then go back to a better format later.

I got it! Thank you!!

[Steven Lord]

Preallocate timeBetween to be a duration array.

timeBetween = hours(zeros(length(dateString), 1));

The reason you receive that error is that MATLAB doesn't know what double precision value a duration like (for example) y = days(1) + hours(2) + minutes(3) + seconds(4) should be converted into. Some valid possibilities include 1.0855 (the number of days y represents), 26.0511 (the number of hours), 1563.1 (the number of minutes), or 93784 (the number of seconds.)

In your example, in order to store the duration value into the double array timeBetween MATLAB needs to perform that conversion. When timeBetween starts off as a duration array, no conversion is necessary.

For simplicity, I used hours to preallocate timeBetween, but you could use minutes, seconds, days, etc. You can also change the Format property of timeBetween after it has been created if you change your mind about how it should be displayed.

[Erin Winkler]

Absolutely perfect. I understand completely. Thank you so very much. :)