# How can I calculate the difference in time (seconds) with datetime?

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Diego on 13 Feb 2014
Commented: Diego on 17 Feb 2014
I have to read a csv file. A column of this file has the next format:
2014/01/16 13:12:12.
Thanks for the help of Mischa Kim I have achieved to read in this format, so this not a problem.
So that my goal is to represent in X axis the time in seconds and Y axis the RSSI related to each second because as you can said me the problem if I represented the TIME_STAMP in the format 13:13:12 there're too many labels If I print all of them.
The First value of TIME_STAMP is 13:12:12 that corresponds to second 1. The Second Value of TIME_STAMP is 13:12:13 that corresponds to second 2. Until the last value...
If you don't understand my question tell me and I'll try to explain better!
Dou you Know a function or a way to calculate the TIME_STAMP in seconds with the aim to represent in x axis the time in seconds with each value.
I attach the csv file.
I would be grateful If you Know how can I do this! Thanks Mischa for your time! Greetings.

Mischa Kim on 13 Feb 2014
If you are starting with date strings, use datenum to convert dates into number (= days) format. As an example:
d2s = 24*3600; % convert from days to seconds
d1 = d2s*datenum('13:12:12');
d2 = d2s*datenum('13:12:16');
display(d2-d1)
So simply loop through the date strings and subtract the first value.
Of course, if you already have dates in double format you would just do subtraction without conversion. It gets even simpler if you know that you have consistent 1 sec spacing between data points. You then just simply
tspan = 1:1:num_datapoints

Mischa Kim on 13 Feb 2014
OK, try this:
T0Str = ['13:12:12';...
'13:12:13';...
'13:12:14'];
d2s = 24*3600; % convert from days to seconds
tspan = zeros(length(T0Str(:,1)),1);
for ii = 1:length(T0Str(:,1))
tspan(ii) = round(d2s*(datenum(T0Str(ii,:))-datenum(T0Str(1,:))));
end
data = rand(3,1); % just same sample data
plot(tspan,data)
set(gca,'XTick',tspan) % remove ticks between data points
Diego on 13 Feb 2014
It works perfect Mischa! Thanks again!
Diego on 14 Feb 2014
One problem that I have is for example when you read increasely the time it works perfect but if an hour is before than the actual hour, the time (in seconds) starts again. For example in this case:
2014/02/10 12:58:20 2014/02/10 12:58:21 2014/02/10 12:57:35

Jos (10584) on 17 Feb 2014
You can also use the dedicated function ETIME (with DATEVEC)
date1 = '2014/01/16 13:12:12'
date2 = '2014/01/16 13:12:13' % one second later
etime(datevec(date2),datevec(date1))
If you have a list of date/times
DT = {'2014/01/16 13:12:13' '2014/01/16 13:12:15' '2014/01/16 13:13:13' '2014/01/17 13:12:13' }
reftime = DT{1} ;
etime(datevec(DT), repmat(datevec(reftime),numel(DT),1))

Diego on 17 Feb 2014
Hi Jos! Thanks for your help!
I don't Know how to pass the string of the time to DT, since I have proved to passed it but it doesn't accept a string of characters. I don't have to write the values manuallity, the code has to put all the values automatically. It is a 100x8 char.
Jos (10584) on 17 Feb 2014
You can convert a N-by-M char array to a N-by-1 cell array of strings of length M, using CELLSTR
A = ['abcd' ; '1234' ; 'xxxx'] % a 3-by-4 char array
B = cellstr(A) % a 3-by-1 cell array of strings
% B{k} equals A(k,:)
Diego on 17 Feb 2014
It's nice Jos! Thanks!!!

sihem chaib on 14 Feb 2014
please how to declare increment beam length in matlab progamme

#### 1 Comment

Diego on 17 Feb 2014
In a vector Sihem?