Tuesday is Equal Pay Day: March 14th represents how far into the year women have had to work to catch up to what their male colleagues earned the previous year.

In other words, women have to work nearly 15 months to earn what men make in 12 months.

82 cents on the dollar, and less for women of color

This is usually referred to as the gender pay gap. Here are the numbers:

- Women earn about 82 cents for every dollar a man earns
- For Black women, it's about 65 cents
- For Latina women, it's about 60 cents

Those gaps widen when comparing what women of color earn to the salaries of White men. These numbers have basically not budged in 20 years. That's particularly strange because so many other things have changed:

- More women now graduate from college than men
- More women graduate from law school than men
- Medical school graduates are roughly half women

That should be seen as progress. So why hasn't the pay gap improved too?

Francine Blau, an economist at Cornell who has been studying the gender pay gap for decades, calls this the $64,000 question. "Although if you adjust for inflation, it's probably in the millions by now," she jokes.

The childcare conundrum

Blau says one of the biggest factors here is childcare. Many women shy away from really demanding positions or work only part time because they need time and flexibility to care for their kids.

"Women will choose jobs or switch to occupations or companies that are more family friendly," she explains. "But a lot of times those jobs will pay less."

Other women leave the workforce entirely. For every woman at a senior management level who gets promoted, two women leave their jobs, most citing childcare as a major reason.

The "unexplained pay gap"

Even if you account for things like women taking more flexible jobs, working fewer hours, taking time off for childcare, etc., paychecks between the sexes still aren't square. Blau and her research partner Lawrence Kahn controlled for "everything we could find reliable data on" and found that women still earn about 8% less than their male colleagues for the same job.

"It's what we call the 'unexplained pay gap,'" says Blau, then laughs. "Or, you could just call it discrimination."

Mend the gap?

One way women could narrow the unexplained pay gap is, of course, to negotiate for higher salaries. But Blau points out that women are likely to experience backlash when they ask for more money. And it can be hard to know how much their male colleagues make and, therefore, what to ask for.

That is changing: a handful of states now require salary ranges be included in job postings.

Blau says that information can be a game changer at work for women and other marginalized groups: "They can get a real sense of, 'Oh, this is the bottom of the range and this is the top of the range. What's reasonable to ask for?'"

A pay raise, if the data is any indication.