If A and B are fermionic operators, and T the time-ordering operator, then the standard definition is
T(AB) = AB, if B precedes A
= - BA, if A precedes B.
Why is there a negative sign? If A and B are space-like separated then it makes sense to assume that A and B anticommute. But...
The electric field in a cubical cavity of side length L with perfectly conducting walls
is
E_x = E_1 cos(n_1 x \pi/L) sin(n_2 y \pi/L) sin(n_3 z \pi/L) sin(\omega t)
E_y = E_2 sin(n_1 x \pi/L) cos(n_2 y \pi/L) sin(n_3 z \pi/L) sin(\omega t)
E_z = E_3 sin(n_1 x \pi/L) sin(n_2 y \pi/L)...