I had severe dieback on Zima Khurma (NB-02) in a frost pocket from our late spring freeze although ti was smaller wood on a frankentree. It survived previous winters of -15ish (maybe -18) with no damage. This thread shows reported cold tolerance of the hybrid varieties and I believe it to be fairly accurate although I think Rosseyanka might be slightly overreported on that list. Maybe add an extra question mark behind it 
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