Humans have recorded the past since ancient times, and to some extent, one of the defining characteristics of human beings is that they are able to record the past, recall it, remember it and confront it with the current state of affairs, thus enabling them to plan accordingly for the future, and to theorise about it as well.
In classical physics the past is just a half of the timeline. In special relativity the past is considered as absolute past or the past cone. In Earth's scale the difference between "classical" and "relativist" past is less than 0.05 s, so it can be neglected in most cases.
In the modern theory of relativity, the conceptual observer is at a geometric point in both space and time at the apex of the 'light cone' which observes events laid out in time as well as space. Different observers can disagree on whether two events at different locations occurred simultaneously depending if the observers are in relative motion (see relativity of simultaneity). This theory depends upon the idea of time as an extended thing and has been confirmed by experiment and has given rise to a philosophical viewpoint known as four dimensionalism. However, although the contents of an observation are time-extended, the conceptual observer, being a geometric point at the origin of the light cone, is not extended in time or space. This analysis contains a paradox in which the conceptual observer contains nothing, even though any real observer would need to be the extended contents of an observation to exist. This paradox is partially resolved in Relativity theory by defining a 'frame of reference' to encompass the measuring instruments used by an observer. This reduces the time separation between instruments to a set of constant intervals.