Select three consecutive dates in a row, column or diagonal.
Write them one beside another to form a single number.
Call it A, the 1st number.
In a similar way select another three dates and write them down to get B,
the 2nd number. Multiply A by B.
Give me the list of digits of the product skipping just
one non-zero digit.
I'll immediately disclose the digit you have skipped.
Why does it work?