Calculate difference between two times

Hello,

I would be very grateful if someone could assist me with the following problem.

I have a data file with one column that supplies dates in the form yyyymmdd and time in another column in the form hhmmss. Please see example below:

20080102 095648

When I read the data into Matlab it is saved in the format Double (i.e. not as a string), e.g.

Time(1:10)

ans =

      183946
      183950
      183954
      183958
      184002

I would like to calculate the time difference between consecutive rows in seconds and then save the results in Double format in an array. I need to account for the fact that the time can go past midnight. However, I am having great difficulty doing doing any date/time calculations because of the format (i.e. not in standard dd/mm/yyyy hh:mm:ss format). It would be great if I could perform the operation using indices, i.e. not in a for loop where I have to subtract each row in sequence.

Thanks in advance for any suggestions.

/Debbie

[Thomas]

where are you getting numbers like 183946 from the format mentioned above (20080102 095648)?

[Debbie_R]

Hi Thomas,

Sorry, I should have been clearer in my question about the data format. The time and date are actually stored in different columns, so the 20080102 095648 shown is just a copy and paste of two adjacent columns from the csv data file. So I read time and date in separately and they are stored in separate arrays in Matlab in Double format (not a string). The Time(1:10) example above is from much further down the data file than the first example (6pm rather than 9am). I was just trying to demonstrate how the data is formatted once read into Matlab - sorry for the confusion.

Many thanks,

Debbie

Answer 1

[Star Strider]

I suggest:

for k1 = 1:size(Date,1)
    DateStr(k1,:) = sprintf('%8d', Date(k1)); 
    TimeStr(k1,:) = sprintf('%06d', Time(k1)); 
    FullDateVct(k1,:) = datevec([DateStr(k1,:)  TimeStr(k1,:)], 'yyyymmddHHMMSS'); 
    FullDateNum(k1) = datenum(FullDateVct(k1,:)); 
end

This is kludgy but when I played around with it, it worked. If all goes well, you only need to do it once. The matrix brackets I used in the 'datevec' call automatically concatanate the strings.

I added FullDateVct to provide readable dates as a check to be sure everything works as you want it to. It's probably best to store the dates and times as a single 'datenum' column and convert them later as necessary.

Then use ‘etime’ to find the difference between two date vectors.

[Debbie_R]

Thank you Star Strider - that worked a treat!

Many thanks,

Debbie

Answer 2

[Thomas]

You can convert the date string into a date number and get the difference between them

EG.

 a  =['20080102 095648';'20080102 105748'];  % two dates apart 1hr 1min
  d=datenum(a,'yyyymmdd HHMMSS');          % convert to number
  difference=d(2)-d(1);                  % difference between the two
  datestr(difference,'HH:MM:SS')     % difference in hr:min:sec

[Debbie_R]

Hi Thomas,

As the data is actually in Double format (not string) and also because the time and date are stored as separate variables (as they are separate columns in the original csv data file), then I guess that I would first need a way to convert the time and date to strings and concatenate them before I could use your solution above. I did try converting the dates to string format, but because I'm trying to do this for many rows, I end up getting something like '200801022008010220080102...' rather than:

20080102

20080102

...

I must be using the incorrect method for conversion.

Many thanks again,

Debbie

[Thomas]

import the two columns fromt he csv files

Eg> dateTime.csv contains

20080102 095648

20080102 105748

20080103 095648

20080103 105748

Then use the following the Matlab script to import and find difference

fid=fopen('dateTime.csv');

data = textscan(fid,'%s %s');

dtTime=strcat(data{:,1}, data{:,2});

d=datenum(dtTime,'yyyymmddHHMMSS');

difference=diff(d);

datestr(difference,'HH:MM:SS')

[Debbie_R]

Hi Thomas,

Thank you very much for this solution.

I am currently using the function importdata to read in the data from the csv file. The file has 3 header lines which have different delimiters to the data part, so I found importdata the easiest way to handle the import and separate out the header lines. This function saves the data in the format MyData.data and when I save Time as, e.g. Time = MyData.Data(:,2), it is formatted as a Double.

Do you know if it would be possible to convert my existing format into a string after the data has been imported and then follow the concatenation method you describe?

Thanks again,

Debbie

[Nathaniel]

The first thing you should do is open Matlab and type:

doc textscan

Then you can use Thomas Anthony's solution above and change:

data=textscan(fid, '%s %s');

to

data=textscan(fid, '%s %s', 'HeaderLines', 3, 'Delimiter', ',');

Answer 3

[Richard]

Its hard to suggest what you can do from your example. I would suggest converting any dates into julian dates when working in matlab and then convert them into a suitable format after. Try using datenum to convert to julian dates:

http://www.mathworks.co.uk/help/techdoc/ref/datenum.html

From your example: when you import the data what appears in your workspace?

Do you have one variable with 20080102 095648 as one entry of a variable or are these divided into two i.e.

time = [20080102, 095648]

If you would provide an example of your data format I would be able to provide more help. I cant see how you get from 20080102 095648 to 183946.

[Debbie_R]

Hi Lestyn,

Thanks for the advice.

The time and date data are provided in a csv file as separate variables. When I import Time I get (for the first 10 rows):

Time(1:10)

ans =

183946

183950

183954

183958

184002

I get exactly the same format for Date, but, e.g. 20081231.

As mentioned in an answer to Thomas above, I selected two different times in my example above (one from 9am and one from 6pm - sorry I should have used more consistent examples).

Many thanks,

Debbie

Answer 4

[Kevin Holst]

since you're getting this from a data file I'd suggest something like this:

fid = fopen('data.txt');
data = textscan(fid,'%s %s');
for i = 1:length(data{1})
    dates(i) = datenum([data{1}{i} 'T' data{2}{i}],'yyyymmddTHHMMSS');
end
difference = diff(dates);
fclose(fid);
vals = str2num(datestr(difference,'SS')); % this assumes that the times will be less than 1 minute apart