The term now is the one used to designate that space of time that happens in the present and that does not extend into the future, nor is it the past. Now is normally considered an instant since everything that happened a second ago is past and everything that will happen after that second is future. However, the notion of now can be very subjectively defined since depending on each situation it can last hours, days or months, for example when it is said that a person's now is the stage in which they settled, formed a family and managed to develop a career of his own.
The notion of now as temporal space is a notion that all human beings, regardless of the culture or civilization to which they belong, have. And that is also what differentiates them from the rest of the animals that do not have a conscious idea about the passage of time, the present, the future or the past. However, each culture and civilization has given a different interpretation to the idea of now, being that for some the present is one more point in a circular succession of time that causes periods that are repeated and that every cycle begins again, while for others the idea of present time is included within the idea of linearity of events.
As said, the idea for now is very subjective and can even vary from situation to situation. This is so because the present can be something durable as well as something momentary, fleeting and very brief. Normally, the idea of now is used to mark the instantaneity or immediacy of something, but in other cases it can also be used as part of a longer and more durable period of time that marks a stage in a person's life.
