While is seems that "Eines Tages" can be used in the future or past, one should look at the context of the examples, where it is used for the past.
I will try to make my point using the same fairy tale as @Takkat [Grimms Märchen: Brüderchen und Schwesterchen]:
Eines Tages hörten sie Hundegebell.
The preceding sentence is:
Tief im Wald kamen sie an ein leerstehendes Haus. Dort lebten sie zufrieden.
This describes the status quo of the story. Then, at some time in the future from that moment, they hear barking dogs, and the story goes on.
While the whole incident was in the past, the sentence "Eines Tages" introduces a later event from that point of the story.
I don't know of any "present" use for "Eines Tages". Any present tense examples could get rephrased as future tense, indeed, "Eines Tages" implies that the event is meant to be in the future, which is common practice in German.