Time representation

Depending on the use cases, time is represented in three different ways in Linux:

1. Wall time (or real time): This is the actual time and date in the real world, such as 07:00 AM, 10 Aug 2017, and is used for timestamps on files and packets sent through the network.
2. Process time: This is the time consumed by a process in its life span. It includes the time consumed by the process in user mode and the time consumed by the kernel code when executing on behalf of the process. This is useful for statistical purposes, auditing, and profiling.
3. Monotonic time: This is the time elapsed since system bootup. It's ever incrementing and monotonic in nature (system uptime).

These three times are measured in either of the following ways:

1. Relative time: This is the time relative to some specific event, such as 7 minutes since system bootup, or 2 minutes since last input from user.
2. Absolute time: This is a unique point in time without any reference to a previous event, such as 10:00 AM, 12 Aug...