Struct Instant

pub struct Instant(/* private fields */);

A measurement of a monotonically nondecreasing clock. Opaque and useful only with Duration.

Instants are always guaranteed, barring platform bugs, to be no less than any previously measured instant when created, and are often useful for tasks such as measuring benchmarks or timing how long an operation takes.

Note, however, that instants are not guaranteed to be steady. In other words, each tick of the underlying clock might not be the same length (e.g. some seconds may be longer than others). An instant may jump forwards or experience time dilation (slow down or speed up), but it will never go backwards. As part of this non-guarantee it is also not specified whether system suspends count as elapsed time or not. The behavior varies across platforms and Rust versions.

Instants are opaque types that can only be compared to one another. There is no method to get “the number of seconds” from an instant. Instead, it only allows measuring the duration between two instants (or comparing two instants).

The size of an Instant struct may vary depending on the target operating system.

Example:

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

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

   // we sleep for 2 seconds
   sleep(Duration::new(2, 0));
   // it prints '2'
   println!("{}", now.elapsed().as_secs());
}

OS-specific behaviors

An Instant is a wrapper around system-specific types and it may behave differently depending on the underlying operating system. For example, the following snippet is fine on Linux but panics on macOS:

use std::time::{Instant, Duration};

let now = Instant::now();
let days_per_10_millennia = 365_2425;
let solar_seconds_per_day = 60 * 60 * 24;
let millennium_in_solar_seconds = 31_556_952_000;
assert_eq!(millennium_in_solar_seconds, days_per_10_millennia * solar_seconds_per_day / 10);

let duration = Duration::new(millennium_in_solar_seconds, 0);
println!("{:?}", now + duration);

For cross-platform code, you can comfortably use durations of up to around one hundred years.

Underlying System calls

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

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.

Monotonicity

On all platforms Instant will try to use an OS API that guarantees monotonic behavior if available, which is the case for all tier 1 platforms. In practice such guarantees are – under rare circumstances – broken by hardware, virtualization or operating system bugs. To work around these bugs and platforms not offering monotonic clocks duration_since, elapsed and sub saturate to zero. In older Rust versions this lead to a panic instead. checked_duration_since can be used to detect and handle situations where monotonicity is violated, or Instants are subtracted in the wrong order.

This workaround obscures programming errors where earlier and later instants are accidentally swapped. For this reason future Rust versions may reintroduce panics.

Implementations

impl Instant

pub fn now() -> Instant

use std::time::Instant;

let now = Instant::now();

pub fn duration_since(&self, earlier: Instant) -> Duration

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

let now = Instant::now();
sleep(Duration::new(1, 0));
let new_now = Instant::now();
println!("{:?}", new_now.duration_since(now));
println!("{:?}", now.duration_since(new_now)); // 0ns

pub fn checked_duration_since(&self, earlier: Instant) -> Option<Duration>

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

let now = Instant::now();
sleep(Duration::new(1, 0));
let new_now = Instant::now();
println!("{:?}", new_now.checked_duration_since(now));
println!("{:?}", now.checked_duration_since(new_now)); // None

pub fn saturating_duration_since(&self, earlier: Instant) -> Duration

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

let now = Instant::now();
sleep(Duration::new(1, 0));
let new_now = Instant::now();
println!("{:?}", new_now.saturating_duration_since(now));
println!("{:?}", now.saturating_duration_since(new_now)); // 0ns

pub fn elapsed(&self) -> Duration

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

let instant = Instant::now();
let three_secs = Duration::from_secs(3);
sleep(three_secs);
assert!(instant.elapsed() >= three_secs);

pub fn checked_add(&self, duration: Duration) -> Option<Instant>

pub fn checked_sub(&self, duration: Duration) -> Option<Instant>

Trait Implementations

impl Add<Duration> for Instant

fn add(self, other: Duration) -> Instant

type Output = Instant

impl AddAssign<Duration> for Instant

fn add_assign(&mut self, other: Duration)

impl Clone for Instant

fn clone(&self) -> Instant

fn clone_from(&mut self, source: &Self)

impl Debug for Instant

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

impl Hash for Instant

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

fn hash_slice<H>(data: &[Self], state: &mut H)where
    H: Hasher,
    Self: Sized,

impl Ord for Instant

fn cmp(&self, other: &Instant) -> Ordering

fn max(self, other: Self) -> Selfwhere
    Self: Sized,

fn min(self, other: Self) -> Selfwhere
    Self: Sized,

fn clamp(self, min: Self, max: Self) -> Selfwhere
    Self: Sized,

impl PartialEq for Instant

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

fn ne(&self, other: &Rhs) -> bool

impl PartialOrd for Instant

fn partial_cmp(&self, other: &Instant) -> Option<Ordering>

fn lt(&self, other: &Rhs) -> bool

fn le(&self, other: &Rhs) -> bool

fn gt(&self, other: &Rhs) -> bool

fn ge(&self, other: &Rhs) -> bool

impl Sub<Duration> for Instant

type Output = Instant

fn sub(self, other: Duration) -> Instant

impl Sub for Instant

fn sub(self, other: Instant) -> Duration

type Output = Duration

impl SubAssign<Duration> for Instant

fn sub_assign(&mut self, other: Duration)

impl Copy for Instant

impl Eq for Instant

impl StructuralPartialEq for Instant

Auto Trait Implementations

impl Freeze for Instant

impl RefUnwindSafe for Instant

impl Send for Instant

impl Sync for Instant

impl Unpin for Instant

impl UnwindSafe for Instant

Blanket Implementations

impl<T> Any for Twhere
    T: 'static + ?Sized,

fn type_id(&self) -> TypeId

impl<T> Borrow<T> for Twhere
    T: ?Sized,

fn borrow(&self) -> &T

impl<T> BorrowMut<T> for Twhere
    T: ?Sized,

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

impl<T> CloneToUninit for Twhere
    T: Clone,

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

impl<T> From<T> for T

fn from(t: T) -> T

impl<T, U> Into<U> for Twhere
    U: From<T>,

fn into(self) -> U

impl<T> ToOwned for Twhere
    T: Clone,

type Owned = T

fn to_owned(&self) -> T

fn clone_into(&self, target: &mut T)

impl<T, U> TryFrom<U> for Twhere
    U: Into<T>,

type Error = Infallible

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

impl<T, U> TryInto<U> for Twhere
    U: TryFrom<T>,

type Error = <U as TryFrom<T>>::Error

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