I'm pretty sure no matter when you reactivate, you pay for that month. I could be wrong, but I had it happen to me a year ago.
I reactivated on, like, one of the last two days of the month.
Got charged the full month.
Though, I was fairly lucky, went to online chat with SE and they actually refunded me because they felt guilty I suppose. [I was surprised.]
[EDIT] Byrth beat me.
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