The demography is a quantitative science that was established on a statistical base, but it has quickly developed its own methods and research elements in response to the particularity of demographic phenomenon and to its real need of specific elements of measuring and analysis that exceed what applied statistics propose. And even if demography meets with many other sciences, yet it has a particularity that results firstly from the effect of time on its events, and secondly from the fact that the demographic events are submitted to a necessary order that must be taken in consideration when we pass from demographic to social analysis of elements and data. This research illuminates this particularity of the necessary order in demographic events, through two access: the first treats time as a demographic concept and how it is related to the necessary order of demographic events; and the second treats the effect of this necessary order in the social analysis of the events that both demography and sociology are interested in, in order to show the importance of gathering demographic and social analysis when treating different demographic-social events, and to illustrate the shortfalls that may affect some models of social analysis if it's not preceded by a demographic analysis of the studied event.