# The Interplay of Time Point, Time Duration, and Clock

This lesson highlights the interplay of time point, time duration, and clock.

This course would not be complete without writing a chapter about the time library. The time library consists of three parts: time point, time duration, and clock; they all depend on each other.



Time point:

The time point is given by its starting point - the so-called epoch - and the time that has elapsed since the epoch (expressed as a time duration)".

Time duration:

The time duration is the difference between two time points. It is measured in the number of time ticks.

Clock:

The clock consists of a starting point and a timer tick. This information enables you to calculate the current time.



You can compare time points. When you add a time duration to a time point, you get a new time point. The time tick is the accuracy of the clock in which you measure the time duration. The birth of Jesus - in my culture - is the starting time point, and a year is a typical time tick.

I will illustrate the three concepts using the lifetime of Dennis Ritchie - the creator of C who died in 2011. For the sake of simplicity, I’m only interested in the years. Here is the lifetime:

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