Phase of harmonics in fft

I took the fft of a signal (i.e sine wave) using fft function. Whilst there is a phase difference of 0.65 between first harmonic and second harmonic. if I want to add only first two harmnonics for an operation what does that 0.65 (rad) phase difference means? Do I need to take it into accout? and how? I know how ever if the phase difference is +/- pi,2pi... the the amplitude of second harmonic will be negative, and if 0 both are having same sign.

[Syed AWM]

I am dealing with max amplitude of each harmonic, so dont know how to take into account a phase difference of 0.65rad