Full Idea
Fand P as 'will' and 'was', G as 'always going to be', H as 'always has been', all tenses reduce to 14 cases: the past series, each implying the next, FH,H,PH,HP,P,GP, and the future series PG,G,FG,GF,F,HF, plus GH=HG implying all, FP=PF which all imply.
Gist of Idea
With four tense operators, all complex tenses reduce to fourteen basic cases
Source
John P. Burgess (Philosophical Logic [2009], 2.8)
Book Reference
Burgess,John P.: 'Philosophical Logic' [Princeton 2009], p.32
A Reaction
I have tried to translate the fourteen into English, but am not quite confident enough to publish them here. I leave it as an exercise for the reader.