The given date was a Sunday.
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6 Jan 1980 was a Sunday.
Monday.
January 6 (of the following year).
Windows built-in calendar says it was a Wednesday.
Consider this. Start on January 9th, and count back the years and leap days (12), and you have 16859 days. 16859 mod 7 = 3, and 9 - 3 = 6, and the 6th was a Wednesday.