Making up an array having same date and time

Hello, I have an array of time and want to compute elapsed time from the first element of the array. For example, I have an array of event arrival times as follows:

arrival_time =

        2015           1           1           0           0           0
        2015           1           1           0           0           6
        2015           1           1           0           0           8
        2015           1           1           0           0           8
        2015           1           1           0           0           9
        2015           1           1           0           0          11
        2015           1           1           0           0          11
        2015           1           1           0           0          12
        2015           1           1           0           0          15
        2015           1           1           0           0          15
        2015           1           1           0           0          16
        2015           1           1           0           0          18
        2015           1           1           0           0          18
        2015           1           1           0           0          18
        2015           1           1           0           0          20
        2015           1           1           0           0          23
        2015           1           1           0           0          23
        2015           1           1           0           0          24
        2015           1           1           0           0          25
        2015           1           1           0           0          26
        2015           1           1           0           0          27
        2015           1           1           0           0          27
        2015           1           1           0           0          29

And I would like to compute elapsed time from arrival_time(1,:).

Question 1. How do I compute the elapsed time easily?

Question 2 . I am going to use the function etime(t2,t1)

t2 = arrival_time(2:end,:);

Then how do I make t1 array that has same element arrival_time(1,:) ?

Many thanks!

Answer 1

[Guillaume]

If you're on 2014b, use datetime objects:

arrival_time = datetime(arrival_time);
ellapsed_time = arrival_time - arrival_time(1);
%to get it in seconds:
ellapsed_seconds = seconds(ellapsed_time);

If you don't have datetime you can do this:

start_time = repmat(arrival_time(1, :), size(arrival_time, 1), 1);
ellapsed_seconds = etime(arrival_time, start_time);

[Yongmin]

Thank you so much for your kind answer!!