the clue is that "when you return" + "by the time you return" both APPEAR to be referring to a future date. Given that the latter part of the sentences are referring to the future, and given that the first part of the sentences will take place at the same time, it follows that the verb should be expressed in the future tense. I'm = I am = PRESENT tense...
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Anonymousother waysI'm not here when you return. No.