'Days past' is the normal English; though I understand the other, it reads oddly. I don't follow your reasoning. Reserve 'passed' for a more clearly verbal situation, like 'Three days passed, and we still felt stiff'.
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Which is correct? Are both correct depending on what you're meaning to say?
i.e. past." This is correct. 'past' describes 'days'.
You see this more clearly if you say '. . . in past days.', which is also correct.'
But note that more common English is to reword, eg This problem is the result of a recent outage.
i.e. "This problem i