Struct SystemTime

struct SystemTime(_)

A measurement of the system clock, useful for talking to external entities like the file system or other processes.

Distinct from the Instant type, this time measurement is not monotonic. This means that you can save a file to the file system, then save another file to the file system, and the second file has a SystemTime measurement earlier than the first. In other words, an operation that happens after another operation in real time may have an earlier SystemTime!

Consequently, comparing two SystemTime instances to learn about the duration between them returns a Result instead of an infallible Duration to indicate that this sort of time drift may happen and needs to be handled.

Although a SystemTime cannot be directly inspected, the UNIX_EPOCH constant is provided in this module as an anchor in time to learn information about a SystemTime. By calculating the duration from this fixed point in time, a SystemTime can be converted to a human-readable time, or perhaps some other string representation.

The size of a SystemTime struct may vary depending on the target operating system.

A SystemTime does not count leap seconds. SystemTime::now()'s behavior around a leap second is the same as the operating system's wall clock. The precise behavior near a leap second (e.g. whether the clock appears to run slow or fast, or stop, or jump) depends on platform and configuration, so should not be relied on.

Example:

use std::time::{Duration, SystemTime};
use std::thread::sleep;

fn main() {
   let now = SystemTime::now();

   // we sleep for 2 seconds
   sleep(Duration::new(2, 0));
   match now.elapsed() {
       Ok(elapsed) => {
           // it prints '2'
           println!("{}", elapsed.as_secs());
       }
       Err(e) => {
           // the system clock went backwards!
           println!("Great Scott! {e:?}");
       }
   }
}

Platform-specific behavior

The precision of SystemTime can depend on the underlying OS-specific time format. For example, on Windows the time is represented in 100 nanosecond intervals whereas Linux can represent nanosecond intervals.

The following system calls are currently being used by now() to find out the current time:

Platform System call
SGX insecure_time usercall. More information on timekeeping in SGX
UNIX clock_gettime (Realtime Clock)
Darwin clock_gettime (Realtime Clock)
VXWorks clock_gettime (Realtime Clock)
SOLID SOLID_RTC_ReadTime
WASI __wasi_clock_time_get (Realtime Clock)
Windows GetSystemTimePreciseAsFileTime / GetSystemTimeAsFileTime

Disclaimer: These system calls might change over time.

Note: mathematical operations like add may panic if the underlying structure cannot represent the new point in time.

Implementations

impl SystemTime

fn now() -> SystemTime

Returns the system time corresponding to "now".

Examples

use std::time::SystemTime;

let sys_time = SystemTime::now();

fn duration_since(self: &Self, earlier: SystemTime) -> Result<Duration, SystemTimeError>

Returns the amount of time elapsed from an earlier point in time.

This function may fail because measurements taken earlier are not guaranteed to always be before later measurements (due to anomalies such as the system clock being adjusted either forwards or backwards). Instant can be used to measure elapsed time without this risk of failure.

If successful, [Ok]([Duration]) is returned where the duration represents the amount of time elapsed from the specified measurement to this one.

Returns an Err if earlier is later than self, and the error contains how far from self the time is.

Examples

use std::time::SystemTime;

let sys_time = SystemTime::now();
let new_sys_time = SystemTime::now();
let difference = new_sys_time.duration_since(sys_time)
    .expect("Clock may have gone backwards");
println!("{difference:?}");

fn elapsed(self: &Self) -> Result<Duration, SystemTimeError>

Returns the difference from this system time to the current clock time.

This function may fail as the underlying system clock is susceptible to drift and updates (e.g., the system clock could go backwards), so this function might not always succeed. If successful, [Ok]([Duration]) is returned where the duration represents the amount of time elapsed from this time measurement to the current time.

To measure elapsed time reliably, use Instant instead.

Returns an Err if self is later than the current system time, and the error contains how far from the current system time self is.

Examples

use std::thread::sleep;
use std::time::{Duration, SystemTime};

let sys_time = SystemTime::now();
let one_sec = Duration::from_secs(1);
sleep(one_sec);
assert!(sys_time.elapsed().unwrap() >= one_sec);

fn checked_add(self: &Self, duration: Duration) -> Option<SystemTime>

Returns Some(t) where t is the time self + duration if t can be represented as SystemTime (which means it's inside the bounds of the underlying data structure), None otherwise.

In the case that the duration is smaller than the time precision of the operating system, Some(self) will be returned.

fn checked_sub(self: &Self, duration: Duration) -> Option<SystemTime>

Returns Some(t) where t is the time self - duration if t can be represented as SystemTime (which means it's inside the bounds of the underlying data structure), None otherwise.

In the case that the duration is smaller than the time precision of the operating system, Some(self) will be returned.

impl Add for SystemTime

fn add(self: Self, dur: Duration) -> SystemTime

Panics

This function may panic if the resulting point in time cannot be represented by the underlying data structure. See SystemTime::checked_add for a version without panic.

impl AddAssign for SystemTime

fn add_assign(self: &mut Self, other: Duration)

impl Clone for SystemTime

fn clone(self: &Self) -> SystemTime

impl Copy for SystemTime

impl Debug for SystemTime

fn fmt(self: &Self, f: &mut fmt::Formatter<'_>) -> fmt::Result

impl Eq for SystemTime

impl Freeze for SystemTime

impl Hash for SystemTime

fn hash<__H: $crate::hash::Hasher>(self: &Self, state: &mut __H)

impl Ord for SystemTime

fn cmp(self: &Self, other: &SystemTime) -> $crate::cmp::Ordering

impl PartialEq for SystemTime

fn eq(self: &Self, other: &SystemTime) -> bool

impl PartialOrd for SystemTime

fn partial_cmp(self: &Self, other: &SystemTime) -> $crate::option::Option<$crate::cmp::Ordering>

impl RefUnwindSafe for SystemTime

impl Send for SystemTime

impl StructuralPartialEq for SystemTime

impl Sub for SystemTime

fn sub(self: Self, dur: Duration) -> SystemTime

impl SubAssign for SystemTime

fn sub_assign(self: &mut Self, other: Duration)

impl Sync for SystemTime

impl Unpin for SystemTime

impl UnwindSafe for SystemTime

impl<T> Any for SystemTime

fn type_id(self: &Self) -> TypeId

impl<T> Borrow for SystemTime

fn borrow(self: &Self) -> &T

impl<T> BorrowMut for SystemTime

fn borrow_mut(self: &mut Self) -> &mut T

impl<T> CloneToUninit for SystemTime

unsafe fn clone_to_uninit(self: &Self, dest: *mut u8)

impl<T> From for SystemTime

fn from(t: T) -> T

Returns the argument unchanged.

impl<T> ToOwned for SystemTime

fn to_owned(self: &Self) -> T
fn clone_into(self: &Self, target: &mut T)

impl<T, U> Into for SystemTime

fn into(self: Self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of [From]<T> for U chooses to do.

impl<T, U> TryFrom for SystemTime

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

impl<T, U> TryInto for SystemTime

fn try_into(self: Self) -> Result<U, <U as TryFrom<T>>::Error>