" However, I would use "has passed" rather than "have passed" because it refers to the duration or period of time, not the two months as if you were counting them. " We wouldn't say,"Ten dollars are a lot of money," because we don't mean the ten separate dollars; we mean the value of the money. This example is not as clear cut as the dollars example, though, and "have" doesn't really seem wrong.
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zbigShould I reformulate the question?Why? I think the first response answers it.