The ways to look at the future and base your future life around is mainly divided up into two categories, short term and long term. While there are definitely gray areas in between, most of what you plan in the future can fall into one of these. Way back in the day when there were cave men and women, survival was a major concern basically 24/7. We didn't have the modern commodities of today like a grocery store or fast food joints to head to whenever we get hungry. Resources for the most part were scarce so by default short term goals became a bigger concern than long term ones. Finding a food source or other necessities was definitely a bigger concern for the average cave person than anything else. In fact I cant really think of any possible long term goals a cave man could have. I'm not really sure though. Who knows, they could in the back of their minds be concerned about settling down with a mate and starting a family with cave children. Regardless however, the most pressing issues dealt with immediate survival, from finding food and shelter to fending off predators. I feel like this trait has been passed down from the generations as humans nowadays are still pretty bad at preparing for the far off future. While we definitely have gotten better at long term planning than our predecessors because we know that we can get food for dinner tonight and aren't worried about being eaten by a lion or something, but we still require time keeping tools like calendars or we would forget a lot of things in our hectic lifestyles.
Without a calendar our civilization would surely crumble under the impending future. If a person didn't mark down on their calendar their next staff meeting was in a month and two days then they will most likely push it to the back of their minds and not be able to recover it in time therefore leading to them being fired. This reminder that got pushed aside was then taken over by more pressing concerns on the individuals mind. As life gets more complicated as technology advances, humans mental capacities just cant keep up with everything. This is why people require the services of a calendar or if you are really important you even can get a personal assistant!!! You know you've made it when somebody's job is to keep tabs on everything dealing with your future and your future engagements. With the use of calendars and setting reminders for our future selves, we are in a way thinking of the long term. Calendars and such keep reminders that are presently long term goals, and help us remember our future short term goals and keep us on top of our lives as time continues to pass us by.