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

When I run an OLS-estimation with three dummy variables yt = 0 + 1t + 2 D2 + 3 D3 + 4 D4 + ut

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