The Constitution, in one of its less explicit sections, mandates that the president shall "from time to time" give Congress his assessment of the state of the union, and offer ideas for what to do about it. Nearly every president has taken this literally, giving either an address (as is now customary and was first done by George Washington) or a written letter (most of the 19th century) to the House and Senate. (The only two who didn't offer state of the union were dead before they had the chance to do so.)

Franklin Roosevelt figured out that this was a pretty good way of talking to the American people, too, and so he introduced the modern, delivered State of the Union address. Looking at the dates of the past addresses, you'll notice that for decades presidents gave their thoughts right at the top of the year. On Monday, House Speaker Paul D. Ryan announced that Obama's 2016 address — which won't necessarily be his last — will be on Jan. 12. That's a relatively late date by the standards of Roosevelt, Truman, Eisenhower, Kennedy and Johnson.

But by modern standards, it's unheard of. Beginning with Richard Nixon, State of the Union addresses moved to late in January. The average dates on which presidents gave the addresses moved from the start of the month to the end. (Every president since Ronald Reagan has also given a speech shortly after inauguration, marked below with hollow circles.) Between 1934 and 1960, there was only one speech later than Jan. 12. Between 1970 and 2015, there were none earlier.

Why the change? It's hard to say. Very early January may conflict more with people's travel and holiday plans, but it's not clear why recent presidents have kicked it so far back. It's clear that more recent presidents are more attentive to when the public is paying attention; only six addresses since 1991 have been on days besides Tuesday — a day when people are more likely to be watching television than, say Friday. (Four of Nixon's five speeches on TV and radio were on Thursday or Friday.)

It's a weird thing to admit after going through all of this data but: The date probably doesn't really matter. We noted earlier this year that viewership of the address keeps slipping downward. The Super Bowl is always far more popular, but the gap is widening.

Maybe, then, recent presidents gave their speeches later in January in order to get more viewers. And maybe, giving the sliding ratings, Obama moved his earlier because it doesn't really matter anyway.