# Derivative using datenum format vector

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Elin Ramström 2019 年 12 月 2 日
Answered: Steven Lord 2019 年 12 月 2 日
Hi everyone,
I am trying to calculate a derivative using (y2-y1)/(x2-x1) thus I am using: gradient(x) ./ gradient(t). The problem is that the vector (t) is in a datenum format therefore when I use the formula the results become incorrect as it do not respect the fact that they in reality are dates.
My intention is to have (y2-y1)/(t2-t1) where the time is in minutes (with respect to seconds as well).
How do I solve this?

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### 採用された回答

Steven Lord 2019 年 12 月 2 日
Use the minutes function to compute the difference between two elements of a datetime array (or two datetime arrays) in terms of a number of minutes. This example is contrived, since I know what m should be, but you could use this with t1 and t2 read in from a file or obtained some other way than computation.
t1 = datetime('now');
dt = randi([30 150]) % 30 to 150 minutes
t2 = t1 + hours(dt/60);
m = minutes(t2-t1) % effectively the same as dt

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