My miles is about to expire at the end of the year.
I am trying to buy some miles so that I can redeem an award.
However, since there is promotion now, buying more miles is cheaper.
If I buy more miles, and spend only a part of the miles purchased (along with the about to expired miles), what is the expiration date of the rest of the miles purchased?

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Edit: The customer service replied my email said it will have two years from the buying date.