The definitions of the word "sunset" revolve around the time of day when the sun sinks below the horizon or, metaphorically, the ending of something else, such as life. Sunset is the actual sinking of the sun, which really just lasts a moment or two.
Sunset, however, is often referred to as a time of day lasting a little longer than the one moment when the sun goes below the horizon. A sunset calls to mind the gathering of clouds and the abundant colors seen around this time of day. The last phases of people's lives are sometimes poetically referred to as the sunsets of their days, and the last years of a particular century are portrayed as the sunset years of that period, indicating finality.