I have 20 quarter observations from 1981:1 to 1990:4

When I run an OLS-estimation with three dummy variables y_t = ß₀ + ß₁×t + ß₂ ×D₂ + ß₃ ×D₃ + ß₄ ×D₄ + u_t

I get a result with the titel  « Model x : OLS, using observations 1986 :2-1990 :2   (T17). »

It is not clear to me why Gretl takes 17 observations instead of 20 to run the OLS estimation.

Thank you in advance

Raul