This dow(day of week) algorithm is based on Sakamoto's methods.
Let's take a look at the Sakamoto's codes.
dayofweek(y, m, d) /* 1 <= m <= 12, y > 1752 (in the U.K.) */
{
static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
y -= m < 3;
return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7;
}
The part of could be
as below,
dayofweek(y, m, d) /* 1 <= m <= 12, y > 1752 (in the U.K.) */
{
static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
y -= m < 3;
return (1.2425 * y + t[m-1] + d) % 7;
}