frequency axis to time axis in power spectral density

Hi all

I have 120 data points for 120 days. and plotted power spectrum as shown in code.. now my doubt is that how to convert frequency axis to time period axis. i defined frequency as

T = number of days (120); f = (0:N/2-1)/T;

then I have plotted power spectrum with frequency axis

then I want to plot power spectrum with time axis..

Now my doubt is period = 1./f; or period = f.*T; ......???? which one is correct.. I was trying to solve this from many days.. some of my friends are telling first is right. some are telling second is right. I am completely lost. somebody help me out..

Thanks in advance

u = xlsread('sample.xlsx');
N= length(u); % number of data points
T = N;
f = (0:N/2-1)/T;        per = f.*T;      period = 1./f;
p_sur_cur= abs(fft(u)/2+1)/(N);           % absolute value of the FFT
pow_sur = p_sur_cur(1:(N/2)).^2;        % take the power of positive freq half
plot(f,pow_sur,'color','r')
figure
plot(period,pow_sur)