I can't prove I'm right

The translation of Machado's: "Al borde del sendero un día nos sentamos."  [At the edge of the path we sit / sat down one day."

Almost all the translators construe this verb as a present tense, but I think it has to be a past.  The form is identical with this verb, but I can't understand what a present tense verb is doing with "un día."  What is the present supposed to be here? It cannot be today, since "un día" is any day except for today.  It cannot be a present of habitual action, since it is "un día," not "todos los días."  So it must be some other kind of present. It could be the historical present ("one day I sit down and ask myself, what am I doing here... etc..."). In this case, though, the past translation works just as well, though. It could be the present used to talk about the immediate future:  "Mañana vamos al cine."  I don't hear this much with "un día," where the future is much more common.

The rest of the poem doesn't help much to decide the issue.

Ya nuestra vida es tiempo, y nuestra sola cuita
son las desesperantes posturas que tomamos
para aguardar ... Mas Ella no faltará a la cita.

[now our life is time, and our only care are the despairing postures we take to wait... But She won't miss  the date.].

I know I'm right, but the translators are against me, and I cannot prove that I'm right.  I tried to google the use of "un día ..." with verbs in the present and found very little, mostly uses of the presented of repeated action.  I found a lot things like "Somos los que un día bebemos Koolroff, Barmons o Velero y al siguiente Absolut."  Willis Barnstone agrees with me at least.