How to find the difference between non-consecutive values

How do I use diff() to find the difference between a certain number of indices? Furthermore, how do I turn the datetime into seconds to find the rate of X over time.

e.g., I have a datetime array t and another array X. I want to find the difference between every two elements of X and find the rate.

t1 = datetime(2013,11,1,8,0,0);
t2 = datetime(2013,11,5,8,0,0);
t = t1:t2

X = 1:5:25

for i = 1:2:length(X)
   A = diff(X(i))/diff(seconds(t(i)))
end

However, I get an empty array in A.

Answer 1

[Voss]

t1 = datetime(2013,11,1,8,0,0);
t2 = datetime(2013,11,5,8,0,0);
t = t1:t2

t = 1x5 datetime array
   01-Nov-2013 08:00:00   02-Nov-2013 08:00:00   03-Nov-2013 08:00:00   04-Nov-2013 08:00:00   05-Nov-2013 08:00:00

X = 1:5:25

X = 1x5

1 6 11 16 21

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A = diff(X(1:2:end))./seconds(diff(t(1:2:end)))

A = 1x2

1.0e-04 * 0.5787 0.5787

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