I'm frustrated by Equal Pay Day.

Yes, I think it's important to point out that the wage gap between men and women still exists, and that a significant chunk of it is unexplained - likely sex discrimination.

Yes, I think using a day in April to symbolize how far into 2010 a woman has to work to match what the average man made in 2009 is a nifty way to get the message across.

But Equal Pay Day targets an outdated version of the problem and obscures one of the primary factors behind the remaining gap.

Today, the big wage gap is between mothers and everyone else.

Before I had my daughter,  I’d never really given much thought to the wage gap between men and women because as a recent college grad, the women I knew usually made as much or more than their husbands or boyfriends or male colleagues.  I reasonably but wrongly concluded that the wage gap issue was a thing of the past.

The typical wage gap figure used to illustrate the wage gap is that women make 77 cents for every dollar a man earns. But that figure is misleading. The wage gap has dramatically closed between young men and women who do not have children. Some data indicates young women with no children now earn 90 percent or more of men's wages.

At the same time, the wage gap is actually widening between mothers and women without children.

Employed mothers earn just 73 percent or less of what men earn. Even less if they are single or minority.

Lest we think this is simply about being a parent, mothers who are employed full-time also earn less than fathers employed full-time.

The wage gap is more and more a mothers’ issue and less and less a women’s issue.

There's another reason the 77% number is too rosy - especially for mothers - because it compares only full-time workers. Women are far more likely than men to work part-time, and when they do, they earn not just less, but less per hour.

Mothers also spend more years out of the workforce than anyone else, usually to care for family. So while typical wage gap data give only a single snapshot in time, the financial impact of mothers’ employment patterns becomes clear only when we look across the years. The lifetime earnings of mothers are just 38 percent of the lifetime earnings of men.

Whoa. Let’s translate that into dollars. If Father earns the median income for men of $43,255.33, over a forty-five year employment lifetime, he’d earn $1,946,475. If Mother earns 38 percent of that over forty-five years, she’d earn $739,660. That’s a gap of more than a million dollars.

Equal Pay Day for women might be today, but mothers would need to keep working two or more weeks to earn what men earned last year.

In other words, the real Equal Pay Day is Mother's Day.

Kristin Maschka is the author of the LA Times Bestseller, This is Not How I Thought It Would Be: Remodeling Motherhood to Get the Lives We Want Today. This piece is cross-posted from her blog.