Instant in std::time - Rust
Description: A measurement of a monotonically nondecreasing clock. Opaque and useful only with `Duration`.

## Instant

# Struct Instant

1.8.0 · Source

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

Expand description

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:

Platform | System call
--- | ---
SGX | `insecure_time` usercall. More information on timekeeping in SGX
UNIX | clock_gettime with `CLOCK_MONOTONIC`
Darwin | clock_gettime with `CLOCK_UPTIME_RAW`
VXWorks | clock_gettime with `CLOCK_MONOTONIC`
SOLID | `get_tim`
WASI | __wasi_clock_time_get with `monotonic`
Windows | QueryPerformanceCounter

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 `Instant`s 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§

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### impl Instant

1.8.0 · Source

#### pub fn now() -> Instant

Returns an instant corresponding to “now”.

##### §Examples

    use std::time::Instant;

    let now = Instant::now();

1.8.0 · Source

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

Returns the amount of time elapsed from another instant to this one, or zero duration if that instant is later than this one.

##### §Panics

Previous Rust versions panicked when `earlier` was later than `self`. Currently this method saturates. Future versions may reintroduce the panic in some circumstances. See Monotonicity.

##### §Examples

    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

1.39.0 · Source

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

Returns the amount of time elapsed from another instant to this one, or None if that instant is later than this one.

Due to monotonicity bugs, even under correct logical ordering of the passed `Instant`s, this method can return `None`.

##### §Examples

    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

1.39.0 · Source

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

Returns the amount of time elapsed from another instant to this one, or zero duration if that instant is later than this one.

##### §Examples

    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

1.8.0 · Source

#### pub fn elapsed(&self) -> Duration

Returns the amount of time elapsed since this instant.

##### §Panics

Previous Rust versions panicked when the current time was earlier than self. Currently this method returns a Duration of zero in that case. Future versions may reintroduce the panic. See Monotonicity.

##### §Examples

    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);

1.34.0 · Source

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

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

1.34.0 · Source

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

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

## Trait Implementations§

1.8.0 · Source§

### impl Add<Duration> for Instant

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

##### §Panics

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

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#### type Output = Instant

The resulting type after applying the `+` operator.
1.9.0 · Source§

### impl AddAssign<Duration> for Instant

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

Performs the `+=` operation. Read more
1.8.0 · Source§

### impl Clone for Instant

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#### fn clone(&self) -> Instant

Returns a duplicate of the value. Read more
1.0.0 · Source§

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

Performs copy-assignment from `source`. Read more
1.8.0 · Source§

### impl Debug for Instant

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

Formats the value using the given formatter. Read more
1.8.0 · Source§

### impl Hash for Instant

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

Feeds this value into the given `Hasher`. Read more
1.3.0 · Source§

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

Feeds a slice of this type into the given `Hasher`. Read more
1.8.0 · Source§

### impl Ord for Instant

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

This method returns an `Ordering` between `self` and `other`. Read more
1.21.0 · Source§

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

Compares and returns the maximum of two values. Read more
1.21.0 · Source§

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

Compares and returns the minimum of two values. Read more
1.50.0 · Source§

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

Restrict a value to a certain interval. Read more
1.8.0 · Source§

### impl PartialEq for Instant

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

Tests for `self` and `other` values to be equal, and is used by `==`.
1.0.0 · Source§

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

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
1.8.0 · Source§

### impl PartialOrd for Instant

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

This method returns an ordering between `self` and `other` values if one exists. Read more
1.0.0 · Source§

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

Tests less than (for `self` and `other`) and is used by the `<` operator. Read more
1.0.0 · Source§

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

Tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more
1.0.0 · Source§

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

Tests greater than (for `self` and `other`) and is used by the `>` operator. Read more
1.0.0 · Source§

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

Tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more
1.8.0 · Source§

### impl Sub<Duration> for Instant

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#### type Output = Instant

The resulting type after applying the `-` operator.
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#### fn sub(self, other: Duration) -> Instant

Performs the `-` operation. Read more
1.8.0 · Source§

### impl Sub for Instant

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

Returns the amount of time elapsed from another instant to this one, or zero duration if that instant is later than this one.

##### §Panics

Previous Rust versions panicked when `other` was later than `self`. Currently this method saturates. Future versions may reintroduce the panic in some circumstances. See Monotonicity.

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#### type Output = Duration

The resulting type after applying the `-` operator.
1.9.0 · Source§

### impl SubAssign<Duration> for Instant

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

Performs the `-=` operation. Read more
1.8.0 · Source§

### impl Copy for Instant

1.8.0 · Source§

### impl Eq for Instant

1.8.0 · Source§

### impl StructuralPartialEq for Instant

## Auto Trait Implementations§

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### impl Freeze for Instant

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### impl RefUnwindSafe for Instant

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### impl Send for Instant

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### impl Sync for Instant

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### impl Unpin for Instant

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### impl UnwindSafe for Instant

## Blanket Implementations§

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

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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

Mutably borrows from an owned value. Read more
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### impl<T> CloneToUninit for T
where T: Clone,

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

🔬This is a nightly-only experimental API. (`clone_to_uninit` #126799)
Performs copy-assignment from `self` to `dest`. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

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

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### impl<T> ToOwned for T
where T: Clone,

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#### type Owned = T

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.