Even though time is continuous, we like to think of think of time in segments.
The most basic unit is the year. Every time the Earth rotates once around the Sun, we take stock of the year gone by, while setting resolutions for the next year. Technically, whether it is 300 or 400 days, time has passed, yet we always mark changes by the number of years. For example, we use age as a marker of what stage of life we should be at (even if it is not a very accurate marker).
Another classic example is the four seasons. The making of accurate calendars and units of time was critical in the development of agriculture as it allowed us to plant many different crops with high efficiency and yield. In older times, people may have marked time by reminiscing what happened around the time of the last harvest while eating food made with ingredients harvested at that time.
Some segments are less regular, such as the last time you caught up with someone, or whenever you have a big life change such as moving cities. Some people may mark the chapters of life by the person they were dating at the time, or the stage of professional development, such as what year of university they were in.
Why do we obsessively divide time and remember our lives in artificial, bite-sized chunks? Our brains are very good at noticing patterns and change, but not so good when the changes are gradual and continuous (change blindness). Ergo, dividing up time into segments help us process the past.
It is hard to notice how much someone’s hair has grown if you see them every day. It is hard to notice how different you and your partner have become with time until a conflict arises. It is hard to notice how much we have grown and matured and changed without consciously reflecting from time to time.
Another interesting thing to consider is that what you use as a marker of time may suggest what you are prioritising. When you look back, what is the unit of time you use to divide your life? Is it the last time you moved cities? Your last boyfriend or girlfriend? Your last big trip? A moment where you came to an epiphany?
Whatever it is, it is a useful practice to periodically look back on all the segments that make up your life, such as by keeping a journal and reading it later. This will let you be more aware of how you are developing as a person and help you steer yourself in the general direction that you want your life to take you. At the very least, it is amusing to notice the subtle differences between the past and the present you.