Obviously all years are arbitrary, and nothing physically changes when Dec 31 1969 becomes Jan 01 1970. But is there a reason to believe that people have put enough meaning into their decade to actually affect the way we write history. I know Bruce Schulman wrote a (fantastic) book called "The Seventies," but do you think professional historians will abandon decades as a unit of analysis? Was there a tradition of historians using decades before the last fifty years or so?
I personally think that decades are useful, but not as a central unit of analysis. Chronology is an important topic of discussion--we always have to justify why we are looking at our sources from Year X to Year Y. But while a decade by itself may not mean much, I do think it often means something to the people who lived through it. That imparts it with historical meaning that, at times, can be useful to draw out.
Not a few historians have referred to the 1890s, for example, as an imperialist decade. Why? Because it can help elucidate the contextual worldview of many ordinary Americans who, in 1898, would enthusiastically support a major departure from U.S. foreign policy modus operandi when the McKinley administration seeks to conquer and annex overseas Spanish territories. Similarly, I tend to think of the 1950s as a cohesive decade, bracketed by powerful anti-communist rhetoric in politics and repression of counterculture. Obviously, the reality is more complex than that, but that framework has been useful at times. The seventies in particular has been subject of a lot of "decade" analysis. Tim Bortelsmann's The 1970s, Daniel Sargent's A Superpower Transformed, and Jeremi Suri's Power and Protest are all great examples of taking a decade and proceeding from the thought, "What the hell happened in those years?" In the case of Bortelsmann, he doesn't look explicitly from Jan 1, 1970 to Dec 31, 1979, but he uses "the seventies" as a shorthand to describe major transformations that take place from 1973 on.
I don't think decade analysis has ever been or will ever be a central unit of analysis, but it is a "crutch" that can be valuable nonetheless. Nostalgia and memory are powerful processes, and have a historical agency on their own. If, for example, millennials think fondly back on the 1990s in U.S. history, a historian would very well consider what the 1990s meant to those who lived through it and think about it after the fact. A historian should still interrogate chronology and question the shorthand of "the nineties, eighties, etc" to try and create a timeline that is more insightful. But it can be a good starting point.