Relation between the phase shift Δϕ and the observation duration T for FRB 180916 by combining Eq. (1) with Eq. (2) in different models. The orange and yellow lines represent an upper limit and the blue dashed line represents a lower limit. The colored area can fit the observation. The solid vertical line shows T = 2.2 yr. The rotation period model does not fit the observation of |Δϕ|≲0.3 radian. The two dashed parallel lines mean Δϕ = 1 radian and Δϕ = 0.3 radian, respectively. These models can be tested by verifying whether or not there will be a detectable phase shift within a longer observation duration.