Hope this helps.
%%Create sample data with 5 Hz sinosoid (Do not include this section in
% your code)
n = 1001;
t = linspace(360,380,n)';
M = [t sin(5*2*pi*t)];
%%Reallocate data
t = M(:,1);
X = M(:,2);
% Trim data (5 seconds before 376.6s)
i5 = t >= 376.6-5 & t <= 376.6;
t = t(i5);
X = X(i5);
Fs = 1/(t(2)-t(1));
%%Calculate Fast Fourier Transform
n = numel(X);
Fn=Fs/2; % Nyquist frequency
Y=fft(X,n); % Calculate fft
fftUnique=Y(1:floor(n/2)); % Find unique values (Symetry)
% Magnitude
fftScale=fftUnique/n; % Scale fft
fftComp=fftScale*2; % Compensate for unique values
mag=abs(fftComp); % Calculate magnitude
% Phase (optional)
phaUnique=angle(fftUnique); % Calculate phase
pha=unwrap(phaUnique); % Correct phase angles to produce
% smoother phase plots
% Frequency
f=(Fn*linspace(0,1,numel(mag)))'; % Output frequency
%%Plot
figure;
subplot(2,1,1)
plot(t,X)
subplot(2,1,2)
plot(f,mag)
