I ran the ETERNA code for the prediction of tidal gravity at some station. Then I use Fourier analysis to recover the amplitude and phase of the S2 constituent. As I understand, the phase of the S2 constituent at mid-night is exactly twice the longitude of the station. But from Fourier transform, I always get the phase which is 1 second less.
The code uses UTC time, and the HW95 frequency catalogue. It does include correction from the difference between UTC and UT1 time. But considering this still does not solve my question.
Is there any other sources of modification of the constituent phase in the code, around 1 second? Thank you in advance!