secrets-of-mental-math-malestrom

Presto-digit-ation: The Art of Mathematical Magic 217
code increases by two, so it will have a year code of 3. The next
year, 2009, is not a leap year, so the year code increases by one,
to 4.
Thus, for example, May 2, 2007, has
Month Code Date Year Code 0 2 1 3
so this date is a Wednesday.
For September 9, 2008, we have
Month Code Date Year Code 4 9 3 16
Subtracting the biggest multiple of 7, we have 16 14 2,
so this date is a Tuesday.
On the other hand, January 16, 2008, is a leap year January,
so its month code will be 5 instead of 6. Thus, we have
Month Code Date Year Code 5 16 3 24
and therefore occurs on day 24 21 3, which is Wednesday.
For your reference, we have listed all of the year codes for the
twenty-first century in the figure on the following page.
The good news is that we do not have to memorize this table.
We can mentally calculate the year code for any date between
2000 and 2099. For the year code of 2000 x, we simply
take the number x/4 (ignoring any remainder) and add that
to x. The year code can be reduced by subtracting any multiple
of 7.
For example, with 2061, we see that 6 1
4 15 (with a remainder
of 1 that we ignore). Thus 2061 has a year code of 61 15
76. And since we can subtract any multiple of 7, we use the simpler
year code of 76 70 6.