Top panel: phase drift of the main peak of the Crab pulsar (changed sign) measured during the observing run in Asiago in October 2008. The (red) curve is the best-fitting parabola (Eq. (2)). The reference epoch t₀ is MJD = 54 749.0, while the reference rotational period is P_init = 0.0336216386529 s. Bottom panel: phase residuals (in μs) after subtracting the best-fitting parabola from the phase-drift.