A month can be an interval of 28, 29, 30, or 31 days. (Look at a calendar.)
640 days =
20.6452 months of 31 days. (rounded)
21.3333 months of 30 days. (rounded)
22.0690 months of 29 days. (rounded)
22.8571 months of 28 days. (rounded)
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