If 09/12/2001(DD/MM/YYYY) happens to be Sunday, then 09/12/1971 would have been a
30 years. The number of leap years is 8 (1972,1976,1980,1984,1988,1992,1996,2000).
So, the total number of days = 22*365 + 8*366 = 10958
10958 mod 7 = 3
Since 9/12/2001 is a Sunday, 9/12/1971 should be a Thursday.
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