System time is the operating system’s view of the current date and time. After boot, the OS takes over from the hardware real-time clock, and applications derive their time values from it. section 9.3 section 9.1
Distinguish system time from nearby timekeeping concepts:
- A real-time clock is a physical, battery-powered motherboard clock that keeps time while the computer is powered off. section 9.3
- System time increments using hardware interrupts and can jump when manually adjusted, so it is not monotonically increasing. section 9.1 section 9.3
- A steady clock is better for measuring durations because it never moves backward, although its values do not represent calendar time. section 9.3
- Absolute, wall-clock, or calendar time expresses the time a person would report, while UTC and TAI provide different standards for representing it. section 9.1 section 9.3
Reading system time in Rust
A small Rust program can ask chrono for the local system time and print the result in a time-zone-aware format:\n\nuse chrono::Local;\n\nfn main() {\n let now = Local::now();\n println!("{}", now);\n}\n\nThe library hides the operating-system and libc interaction while returning a typed value containing a time zone. Time-zone-free timestamps and illegal time arithmetic are rejected at compile time. section 9.5
Figure 9.1 An application gets time information from the OS, usually functionally provided by the system’s libc implementation.
Trace how an application obtains time information from the operating system, usually through the system’s libc implementation. section 9.5
