Tom's birthday must be in the next year; otherwise, if you reverse the month and day, it will be a few days less than one or two months, and not 42 days. (For example, 2.1 is 30 days after 1.2, and 5.3 is 59 days after 3.5.)

If Tom's birthday is in the next year, it's easy to see that it must be on January 12, and today is December 1st.
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