So the effect you want is that the "Impervious" extra on your Toughness save is limited to only during the day, or something similar?
That seems a little "meta" to me - it's like wanting to apply an extra to Protection only under certain circumstances. You're wanting to apply something like the "Limited" power flaw, but you want to apply it to the Extra, not the power itself. As I understand it, that's not something the rules support.
I think the best solution here is to create a new extra, maybe with a cost of 0.5, that acts like the Impervious extra but only works half the time. That way you can apply your new extra to the "(Saving Throw) Protection" power, or a regular Protection power, and have it work appropriately.
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