April 8 is "Equal Pay Day," an annual event to raise awareness regarding the so-called gender wage gap. As President Obama said in the State of the Union address, women "still make 77 cents for every dollar a man earns," a claim echoed by the National Committee on Pay Equity, the American Association of University Women and other progressive groups.

The 23% gap implies that women work an extra 68 days to earn the same pay as a man. Mr. Obama advocates allowing women to sue for wage discrimination, with...