I'm creating a space opera (or a soft science fiction in space) universe, but I am trying to devise a calendar and I don't know if I should use the calendar of Earth (using the time before and after of Christ) or a fictional calendar (using another dates).
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Short answer: It's your story, you can do whatever you like!
Unless this is meant to be a historically accurate recounting of actual events, then you can change as little or as much as you like. It's fiction, so you can choose to have it almost exactly the same as reality with a couple of things changing, or you can choose to have it completely unlike reality, both with respect to history or to physics.
It can be based on real events, but the explanations can be supernatural or science fiction or real events can be altered in any way that helps progress your story.
You have already indicated that it's meant to be soft science fiction, which gives you as much latitude as you need to change reality in whatever way suits your story best :)