Measuring Phase Difference at DC bin aliased frequency using FFT
The "DC bin" does not represent DC exclusively, but has also some low frequency energy. How much depends on the fft size and sampling rate. Simple example: if your fft size is 1000 and your sampling rate is 1000Hz, then the dc bin represents frequencies from 0 to 0.5Hz, as the distance between fft bins is 1Hz. So, a phase can be calculated for the DC bin.