1 //! Temporal quantification.
5 //! There are multiple ways to create a new [`Duration`]:
8 //! # use std::time::Duration;
9 //! let five_seconds = Duration::from_secs(5);
10 //! assert_eq!(five_seconds, Duration::from_millis(5_000));
11 //! assert_eq!(five_seconds, Duration::from_micros(5_000_000));
12 //! assert_eq!(five_seconds, Duration::from_nanos(5_000_000_000));
14 //! let ten_seconds = Duration::from_secs(10);
15 //! let seven_nanos = Duration::from_nanos(7);
16 //! let total = ten_seconds + seven_nanos;
17 //! assert_eq!(total, Duration::new(10, 7));
20 //! Using [`Instant`] to calculate how long a function took to run:
22 //! ```ignore (incomplete)
23 //! let now = Instant::now();
25 //! // Calling a slow function, it may take a while
28 //! let elapsed_time = now.elapsed();
29 //! println!("Running slow_function() took {} seconds.", elapsed_time.as_secs());
32 #![stable(feature = "time", since = "1.3.0")]
38 use crate::error::Error;
40 use crate::ops::{Add, AddAssign, Sub, SubAssign};
42 use crate::sys_common::FromInner;
44 #[stable(feature = "time", since = "1.3.0")]
45 pub use core::time::Duration;
47 #[unstable(feature = "duration_checked_float", issue = "83400")]
48 pub use core::time::FromSecsError;
50 /// A measurement of a monotonically nondecreasing clock.
51 /// Opaque and useful only with [`Duration`].
53 /// Instants are always guaranteed to be no less than any previously measured
54 /// instant when created, and are often useful for tasks such as measuring
55 /// benchmarks or timing how long an operation takes.
57 /// Note, however, that instants are not guaranteed to be **steady**. In other
58 /// words, each tick of the underlying clock might not be the same length (e.g.
59 /// some seconds may be longer than others). An instant may jump forwards or
60 /// experience time dilation (slow down or speed up), but it will never go
63 /// Instants are opaque types that can only be compared to one another. There is
64 /// no method to get "the number of seconds" from an instant. Instead, it only
65 /// allows measuring the duration between two instants (or comparing two
68 /// The size of an `Instant` struct may vary depending on the target operating
74 /// use std::time::{Duration, Instant};
75 /// use std::thread::sleep;
78 /// let now = Instant::now();
80 /// // we sleep for 2 seconds
81 /// sleep(Duration::new(2, 0));
83 /// println!("{}", now.elapsed().as_secs());
87 /// # OS-specific behaviors
89 /// An `Instant` is a wrapper around system-specific types and it may behave
90 /// differently depending on the underlying operating system. For example,
91 /// the following snippet is fine on Linux but panics on macOS:
94 /// use std::time::{Instant, Duration};
96 /// let now = Instant::now();
97 /// let max_nanoseconds = u64::MAX / 1_000_000_000;
98 /// let duration = Duration::new(max_nanoseconds, 0);
99 /// println!("{:?}", now + duration);
102 /// # Underlying System calls
103 /// Currently, the following system calls are being used to get the current time using `now()`:
105 /// | Platform | System call |
106 /// |-----------|----------------------------------------------------------------------|
107 /// | SGX | [`insecure_time` usercall]. More information on [timekeeping in SGX] |
108 /// | UNIX | [clock_gettime (Monotonic Clock)] |
109 /// | Darwin | [mach_absolute_time] |
110 /// | VXWorks | [clock_gettime (Monotonic Clock)] |
111 /// | SOLID | `get_tim` |
112 /// | WASI | [__wasi_clock_time_get (Monotonic Clock)] |
113 /// | Windows | [QueryPerformanceCounter] |
115 /// [QueryPerformanceCounter]: https://docs.microsoft.com/en-us/windows/win32/api/profileapi/nf-profileapi-queryperformancecounter
116 /// [`insecure_time` usercall]: https://edp.fortanix.com/docs/api/fortanix_sgx_abi/struct.Usercalls.html#method.insecure_time
117 /// [timekeeping in SGX]: https://edp.fortanix.com/docs/concepts/rust-std/#codestdtimecode
118 /// [__wasi_clock_time_get (Monotonic Clock)]: https://github.com/WebAssembly/WASI/blob/master/phases/snapshot/docs.md#clock_time_get
119 /// [clock_gettime (Monotonic Clock)]: https://linux.die.net/man/3/clock_gettime
120 /// [mach_absolute_time]: https://developer.apple.com/library/archive/documentation/Darwin/Conceptual/KernelProgramming/services/services.html
122 /// **Disclaimer:** These system calls might change over time.
124 /// > Note: mathematical operations like [`add`] may panic if the underlying
125 /// > structure cannot represent the new point in time.
127 /// [`add`]: Instant::add
128 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
129 #[stable(feature = "time2", since = "1.8.0")]
130 pub struct Instant(time::Instant);
132 /// A measurement of the system clock, useful for talking to
133 /// external entities like the file system or other processes.
135 /// Distinct from the [`Instant`] type, this time measurement **is not
136 /// monotonic**. This means that you can save a file to the file system, then
137 /// save another file to the file system, **and the second file has a
138 /// `SystemTime` measurement earlier than the first**. In other words, an
139 /// operation that happens after another operation in real time may have an
140 /// earlier `SystemTime`!
142 /// Consequently, comparing two `SystemTime` instances to learn about the
143 /// duration between them returns a [`Result`] instead of an infallible [`Duration`]
144 /// to indicate that this sort of time drift may happen and needs to be handled.
146 /// Although a `SystemTime` cannot be directly inspected, the [`UNIX_EPOCH`]
147 /// constant is provided in this module as an anchor in time to learn
148 /// information about a `SystemTime`. By calculating the duration from this
149 /// fixed point in time, a `SystemTime` can be converted to a human-readable time,
150 /// or perhaps some other string representation.
152 /// The size of a `SystemTime` struct may vary depending on the target operating
158 /// use std::time::{Duration, SystemTime};
159 /// use std::thread::sleep;
162 /// let now = SystemTime::now();
164 /// // we sleep for 2 seconds
165 /// sleep(Duration::new(2, 0));
166 /// match now.elapsed() {
169 /// println!("{}", elapsed.as_secs());
172 /// // an error occurred!
173 /// println!("Error: {:?}", e);
179 /// # Underlying System calls
180 /// Currently, the following system calls are being used to get the current time using `now()`:
182 /// | Platform | System call |
183 /// |-----------|----------------------------------------------------------------------|
184 /// | SGX | [`insecure_time` usercall]. More information on [timekeeping in SGX] |
185 /// | UNIX | [clock_gettime (Realtime Clock)] |
186 /// | Darwin | [gettimeofday] |
187 /// | VXWorks | [clock_gettime (Realtime Clock)] |
188 /// | SOLID | `SOLID_RTC_ReadTime` |
189 /// | WASI | [__wasi_clock_time_get (Realtime Clock)] |
190 /// | Windows | [GetSystemTimePreciseAsFileTime] / [GetSystemTimeAsFileTime] |
192 /// [`insecure_time` usercall]: https://edp.fortanix.com/docs/api/fortanix_sgx_abi/struct.Usercalls.html#method.insecure_time
193 /// [timekeeping in SGX]: https://edp.fortanix.com/docs/concepts/rust-std/#codestdtimecode
194 /// [gettimeofday]: https://man7.org/linux/man-pages/man2/gettimeofday.2.html
195 /// [clock_gettime (Realtime Clock)]: https://linux.die.net/man/3/clock_gettime
196 /// [__wasi_clock_time_get (Realtime Clock)]: https://github.com/WebAssembly/WASI/blob/master/phases/snapshot/docs.md#clock_time_get
197 /// [GetSystemTimePreciseAsFileTime]: https://docs.microsoft.com/en-us/windows/win32/api/sysinfoapi/nf-sysinfoapi-getsystemtimepreciseasfiletime
198 /// [GetSystemTimeAsFileTime]: https://docs.microsoft.com/en-us/windows/win32/api/sysinfoapi/nf-sysinfoapi-getsystemtimeasfiletime
200 /// **Disclaimer:** These system calls might change over time.
202 /// > Note: mathematical operations like [`add`] may panic if the underlying
203 /// > structure cannot represent the new point in time.
205 /// [`add`]: SystemTime::add
206 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
207 #[stable(feature = "time2", since = "1.8.0")]
208 pub struct SystemTime(time::SystemTime);
210 /// An error returned from the `duration_since` and `elapsed` methods on
211 /// `SystemTime`, used to learn how far in the opposite direction a system time
217 /// use std::thread::sleep;
218 /// use std::time::{Duration, SystemTime};
220 /// let sys_time = SystemTime::now();
221 /// sleep(Duration::from_secs(1));
222 /// let new_sys_time = SystemTime::now();
223 /// match sys_time.duration_since(new_sys_time) {
225 /// Err(e) => println!("SystemTimeError difference: {:?}", e.duration()),
228 #[derive(Clone, Debug)]
229 #[stable(feature = "time2", since = "1.8.0")]
230 pub struct SystemTimeError(Duration);
233 /// Returns an instant corresponding to "now".
238 /// use std::time::Instant;
240 /// let now = Instant::now();
242 #[stable(feature = "time2", since = "1.8.0")]
243 pub fn now() -> Instant {
244 let os_now = time::Instant::now();
246 // And here we come upon a sad state of affairs. The whole point of
247 // `Instant` is that it's monotonically increasing. We've found in the
248 // wild, however, that it's not actually monotonically increasing for
249 // one reason or another. These appear to be OS and hardware level bugs,
250 // and there's not really a whole lot we can do about them. Here's a
251 // taste of what we've found:
253 // * #48514 - OpenBSD, x86_64
254 // * #49281 - linux arm64 and s390x
255 // * #51648 - windows, x86
256 // * #56560 - windows, x86_64, AWS
257 // * #56612 - windows, x86, vm (?)
258 // * #56940 - linux, arm64
259 // * https://bugzilla.mozilla.org/show_bug.cgi?id=1487778 - a similar
262 // It seems that this just happens a lot in the wild.
263 // We're seeing panics across various platforms where consecutive calls
264 // to `Instant::now`, such as via the `elapsed` function, are panicking
265 // as they're going backwards. Placed here is a last-ditch effort to try
266 // to fix things up. We keep a global "latest now" instance which is
267 // returned instead of what the OS says if the OS goes backwards.
269 // To hopefully mitigate the impact of this, a few platforms are
270 // excluded as "these at least haven't gone backwards yet".
272 // While issues have been seen on arm64 platforms the Arm architecture
273 // requires that the counter monotonically increases and that it must
274 // provide a uniform view of system time (e.g. it must not be possible
275 // for a core to recieve a message from another core with a time stamp
276 // and observe time going backwards (ARM DDI 0487G.b D11.1.2). While
277 // there have been a few 64bit SoCs that have bugs which cause time to
278 // not monoticially increase, these have been fixed in the Linux kernel
279 // and we shouldn't penalize all Arm SoCs for those who refuse to
280 // update their kernels:
281 // SUN50I_ERRATUM_UNKNOWN1 - Allwinner A64 / Pine A64 - fixed in 5.1
282 // FSL_ERRATUM_A008585 - Freescale LS2080A/LS1043A - fixed in 4.10
283 // HISILICON_ERRATUM_161010101 - Hisilicon 1610 - fixed in 4.11
284 // ARM64_ERRATUM_858921 - Cortex A73 - fixed in 4.12
285 if time::Instant::actually_monotonic() {
286 return Instant(os_now);
289 Instant(monotonic::monotonize(os_now))
292 /// Returns the amount of time elapsed from another instant to this one.
296 /// This function will panic if `earlier` is later than `self`.
301 /// use std::time::{Duration, Instant};
302 /// use std::thread::sleep;
304 /// let now = Instant::now();
305 /// sleep(Duration::new(1, 0));
306 /// let new_now = Instant::now();
307 /// println!("{:?}", new_now.duration_since(now));
309 #[stable(feature = "time2", since = "1.8.0")]
310 pub fn duration_since(&self, earlier: Instant) -> Duration {
311 self.0.checked_sub_instant(&earlier.0).expect("supplied instant is later than self")
314 /// Returns the amount of time elapsed from another instant to this one,
315 /// or None if that instant is later than this one.
320 /// use std::time::{Duration, Instant};
321 /// use std::thread::sleep;
323 /// let now = Instant::now();
324 /// sleep(Duration::new(1, 0));
325 /// let new_now = Instant::now();
326 /// println!("{:?}", new_now.checked_duration_since(now));
327 /// println!("{:?}", now.checked_duration_since(new_now)); // None
329 #[stable(feature = "checked_duration_since", since = "1.39.0")]
330 pub fn checked_duration_since(&self, earlier: Instant) -> Option<Duration> {
331 self.0.checked_sub_instant(&earlier.0)
334 /// Returns the amount of time elapsed from another instant to this one,
335 /// or zero duration if that instant is later than this one.
340 /// use std::time::{Duration, Instant};
341 /// use std::thread::sleep;
343 /// let now = Instant::now();
344 /// sleep(Duration::new(1, 0));
345 /// let new_now = Instant::now();
346 /// println!("{:?}", new_now.saturating_duration_since(now));
347 /// println!("{:?}", now.saturating_duration_since(new_now)); // 0ns
349 #[stable(feature = "checked_duration_since", since = "1.39.0")]
350 pub fn saturating_duration_since(&self, earlier: Instant) -> Duration {
351 self.checked_duration_since(earlier).unwrap_or_default()
354 /// Returns the amount of time elapsed since this instant was created.
358 /// This function may panic if the current time is earlier than this
359 /// instant, which is something that can happen if an `Instant` is
360 /// produced synthetically.
365 /// use std::thread::sleep;
366 /// use std::time::{Duration, Instant};
368 /// let instant = Instant::now();
369 /// let three_secs = Duration::from_secs(3);
370 /// sleep(three_secs);
371 /// assert!(instant.elapsed() >= three_secs);
373 #[stable(feature = "time2", since = "1.8.0")]
374 pub fn elapsed(&self) -> Duration {
375 Instant::now() - *self
378 /// Returns `Some(t)` where `t` is the time `self + duration` if `t` can be represented as
379 /// `Instant` (which means it's inside the bounds of the underlying data structure), `None`
381 #[stable(feature = "time_checked_add", since = "1.34.0")]
382 pub fn checked_add(&self, duration: Duration) -> Option<Instant> {
383 self.0.checked_add_duration(&duration).map(Instant)
386 /// Returns `Some(t)` where `t` is the time `self - duration` if `t` can be represented as
387 /// `Instant` (which means it's inside the bounds of the underlying data structure), `None`
389 #[stable(feature = "time_checked_add", since = "1.34.0")]
390 pub fn checked_sub(&self, duration: Duration) -> Option<Instant> {
391 self.0.checked_sub_duration(&duration).map(Instant)
395 #[stable(feature = "time2", since = "1.8.0")]
396 impl Add<Duration> for Instant {
397 type Output = Instant;
401 /// This function may panic if the resulting point in time cannot be represented by the
402 /// underlying data structure. See [`Instant::checked_add`] for a version without panic.
403 fn add(self, other: Duration) -> Instant {
404 self.checked_add(other).expect("overflow when adding duration to instant")
408 #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
409 impl AddAssign<Duration> for Instant {
410 fn add_assign(&mut self, other: Duration) {
411 *self = *self + other;
415 #[stable(feature = "time2", since = "1.8.0")]
416 impl Sub<Duration> for Instant {
417 type Output = Instant;
419 fn sub(self, other: Duration) -> Instant {
420 self.checked_sub(other).expect("overflow when subtracting duration from instant")
424 #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
425 impl SubAssign<Duration> for Instant {
426 fn sub_assign(&mut self, other: Duration) {
427 *self = *self - other;
431 #[stable(feature = "time2", since = "1.8.0")]
432 impl Sub<Instant> for Instant {
433 type Output = Duration;
435 fn sub(self, other: Instant) -> Duration {
436 self.duration_since(other)
440 #[stable(feature = "time2", since = "1.8.0")]
441 impl fmt::Debug for Instant {
442 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
448 /// An anchor in time which can be used to create new `SystemTime` instances or
449 /// learn about where in time a `SystemTime` lies.
451 /// This constant is defined to be "1970-01-01 00:00:00 UTC" on all systems with
452 /// respect to the system clock. Using `duration_since` on an existing
453 /// `SystemTime` instance can tell how far away from this point in time a
454 /// measurement lies, and using `UNIX_EPOCH + duration` can be used to create a
455 /// `SystemTime` instance to represent another fixed point in time.
460 /// use std::time::SystemTime;
462 /// match SystemTime::now().duration_since(SystemTime::UNIX_EPOCH) {
463 /// Ok(n) => println!("1970-01-01 00:00:00 UTC was {} seconds ago!", n.as_secs()),
464 /// Err(_) => panic!("SystemTime before UNIX EPOCH!"),
467 #[stable(feature = "assoc_unix_epoch", since = "1.28.0")]
468 pub const UNIX_EPOCH: SystemTime = UNIX_EPOCH;
470 /// Returns the system time corresponding to "now".
475 /// use std::time::SystemTime;
477 /// let sys_time = SystemTime::now();
479 #[stable(feature = "time2", since = "1.8.0")]
480 pub fn now() -> SystemTime {
481 SystemTime(time::SystemTime::now())
484 /// Returns the amount of time elapsed from an earlier point in time.
486 /// This function may fail because measurements taken earlier are not
487 /// guaranteed to always be before later measurements (due to anomalies such
488 /// as the system clock being adjusted either forwards or backwards).
489 /// [`Instant`] can be used to measure elapsed time without this risk of failure.
491 /// If successful, <code>[Ok]\([Duration])</code> is returned where the duration represents
492 /// the amount of time elapsed from the specified measurement to this one.
494 /// Returns an [`Err`] if `earlier` is later than `self`, and the error
495 /// contains how far from `self` the time is.
500 /// use std::time::SystemTime;
502 /// let sys_time = SystemTime::now();
503 /// let new_sys_time = SystemTime::now();
504 /// let difference = new_sys_time.duration_since(sys_time)
505 /// .expect("Clock may have gone backwards");
506 /// println!("{:?}", difference);
508 #[stable(feature = "time2", since = "1.8.0")]
509 pub fn duration_since(&self, earlier: SystemTime) -> Result<Duration, SystemTimeError> {
510 self.0.sub_time(&earlier.0).map_err(SystemTimeError)
513 /// Returns the difference between the clock time when this
514 /// system time was created, and the current clock time.
516 /// This function may fail as the underlying system clock is susceptible to
517 /// drift and updates (e.g., the system clock could go backwards), so this
518 /// function might not always succeed. If successful, <code>[Ok]\([Duration])</code> is
519 /// returned where the duration represents the amount of time elapsed from
520 /// this time measurement to the current time.
522 /// To measure elapsed time reliably, use [`Instant`] instead.
524 /// Returns an [`Err`] if `self` is later than the current system time, and
525 /// the error contains how far from the current system time `self` is.
530 /// use std::thread::sleep;
531 /// use std::time::{Duration, SystemTime};
533 /// let sys_time = SystemTime::now();
534 /// let one_sec = Duration::from_secs(1);
536 /// assert!(sys_time.elapsed().unwrap() >= one_sec);
538 #[stable(feature = "time2", since = "1.8.0")]
539 pub fn elapsed(&self) -> Result<Duration, SystemTimeError> {
540 SystemTime::now().duration_since(*self)
543 /// Returns `Some(t)` where `t` is the time `self + duration` if `t` can be represented as
544 /// `SystemTime` (which means it's inside the bounds of the underlying data structure), `None`
546 #[stable(feature = "time_checked_add", since = "1.34.0")]
547 pub fn checked_add(&self, duration: Duration) -> Option<SystemTime> {
548 self.0.checked_add_duration(&duration).map(SystemTime)
551 /// Returns `Some(t)` where `t` is the time `self - duration` if `t` can be represented as
552 /// `SystemTime` (which means it's inside the bounds of the underlying data structure), `None`
554 #[stable(feature = "time_checked_add", since = "1.34.0")]
555 pub fn checked_sub(&self, duration: Duration) -> Option<SystemTime> {
556 self.0.checked_sub_duration(&duration).map(SystemTime)
560 #[stable(feature = "time2", since = "1.8.0")]
561 impl Add<Duration> for SystemTime {
562 type Output = SystemTime;
566 /// This function may panic if the resulting point in time cannot be represented by the
567 /// underlying data structure. See [`SystemTime::checked_add`] for a version without panic.
568 fn add(self, dur: Duration) -> SystemTime {
569 self.checked_add(dur).expect("overflow when adding duration to instant")
573 #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
574 impl AddAssign<Duration> for SystemTime {
575 fn add_assign(&mut self, other: Duration) {
576 *self = *self + other;
580 #[stable(feature = "time2", since = "1.8.0")]
581 impl Sub<Duration> for SystemTime {
582 type Output = SystemTime;
584 fn sub(self, dur: Duration) -> SystemTime {
585 self.checked_sub(dur).expect("overflow when subtracting duration from instant")
589 #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
590 impl SubAssign<Duration> for SystemTime {
591 fn sub_assign(&mut self, other: Duration) {
592 *self = *self - other;
596 #[stable(feature = "time2", since = "1.8.0")]
597 impl fmt::Debug for SystemTime {
598 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
603 /// An anchor in time which can be used to create new `SystemTime` instances or
604 /// learn about where in time a `SystemTime` lies.
606 /// This constant is defined to be "1970-01-01 00:00:00 UTC" on all systems with
607 /// respect to the system clock. Using `duration_since` on an existing
608 /// [`SystemTime`] instance can tell how far away from this point in time a
609 /// measurement lies, and using `UNIX_EPOCH + duration` can be used to create a
610 /// [`SystemTime`] instance to represent another fixed point in time.
615 /// use std::time::{SystemTime, UNIX_EPOCH};
617 /// match SystemTime::now().duration_since(UNIX_EPOCH) {
618 /// Ok(n) => println!("1970-01-01 00:00:00 UTC was {} seconds ago!", n.as_secs()),
619 /// Err(_) => panic!("SystemTime before UNIX EPOCH!"),
622 #[stable(feature = "time2", since = "1.8.0")]
623 pub const UNIX_EPOCH: SystemTime = SystemTime(time::UNIX_EPOCH);
625 impl SystemTimeError {
626 /// Returns the positive duration which represents how far forward the
627 /// second system time was from the first.
629 /// A `SystemTimeError` is returned from the [`SystemTime::duration_since`]
630 /// and [`SystemTime::elapsed`] methods whenever the second system time
631 /// represents a point later in time than the `self` of the method call.
636 /// use std::thread::sleep;
637 /// use std::time::{Duration, SystemTime};
639 /// let sys_time = SystemTime::now();
640 /// sleep(Duration::from_secs(1));
641 /// let new_sys_time = SystemTime::now();
642 /// match sys_time.duration_since(new_sys_time) {
644 /// Err(e) => println!("SystemTimeError difference: {:?}", e.duration()),
647 #[stable(feature = "time2", since = "1.8.0")]
648 pub fn duration(&self) -> Duration {
653 #[stable(feature = "time2", since = "1.8.0")]
654 impl Error for SystemTimeError {
656 fn description(&self) -> &str {
657 "other time was not earlier than self"
661 #[stable(feature = "time2", since = "1.8.0")]
662 impl fmt::Display for SystemTimeError {
663 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
664 write!(f, "second time provided was later than self")
668 impl FromInner<time::SystemTime> for SystemTime {
669 fn from_inner(time: time::SystemTime) -> SystemTime {