matlab time seires cpdomg

Heres what I Have sofar

outdoorapr = xlsread('outdoorapr.xlsx','D2:D8586');

indoorapr=xlsread('indoorapr.xlsx','D2:D8586');

time=xlsread('time.xlsx','C2:C8586')

t=time

a=(outdoorapr);

b=(indoorapr);

dt=diff(a)/5;

c=dt./(b(1:end-1)-a(1:end-1));%diff(a) will be one less than matrix a and b

plot(t,c);

I keep running into errors, I understand that I need to code the time as a function but idk how to? My time isnt uniform so I'd prefer using seconds

I dk how to read the data as text