1 //! Temporal quantification.
6 //! use std::time::Duration;
8 //! let five_seconds = Duration::new(5, 0);
9 //! // both declarations are equivalent
10 //! assert_eq!(Duration::new(5, 0), Duration::from_secs(5));
13 #![stable(feature = "time", since = "1.3.0")]
16 use crate::error::Error;
18 use crate::ops::{Add, Sub, AddAssign, SubAssign};
20 use crate::sys_common::FromInner;
21 use crate::sys_common::mutex::Mutex;
23 #[stable(feature = "time", since = "1.3.0")]
24 pub use core::time::Duration;
26 /// A measurement of a monotonically nondecreasing clock.
27 /// Opaque and useful only with `Duration`.
29 /// Instants are always guaranteed to be no less than any previously measured
30 /// instant when created, and are often useful for tasks such as measuring
31 /// benchmarks or timing how long an operation takes.
33 /// Note, however, that instants are not guaranteed to be **steady**. In other
34 /// words, each tick of the underlying clock may not be the same length (e.g.
35 /// some seconds may be longer than others). An instant may jump forwards or
36 /// experience time dilation (slow down or speed up), but it will never go
39 /// Instants are opaque types that can only be compared to one another. There is
40 /// no method to get "the number of seconds" from an instant. Instead, it only
41 /// allows measuring the duration between two instants (or comparing two
44 /// The size of an `Instant` struct may vary depending on the target operating
50 /// use std::time::{Duration, Instant};
51 /// use std::thread::sleep;
54 /// let now = Instant::now();
56 /// // we sleep for 2 seconds
57 /// sleep(Duration::new(2, 0));
59 /// println!("{}", now.elapsed().as_secs());
62 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
63 #[stable(feature = "time2", since = "1.8.0")]
64 pub struct Instant(time::Instant);
66 /// A measurement of the system clock, useful for talking to
67 /// external entities like the file system or other processes.
69 /// Distinct from the [`Instant`] type, this time measurement **is not
70 /// monotonic**. This means that you can save a file to the file system, then
71 /// save another file to the file system, **and the second file has a
72 /// `SystemTime` measurement earlier than the first**. In other words, an
73 /// operation that happens after another operation in real time may have an
74 /// earlier `SystemTime`!
76 /// Consequently, comparing two `SystemTime` instances to learn about the
77 /// duration between them returns a [`Result`] instead of an infallible [`Duration`]
78 /// to indicate that this sort of time drift may happen and needs to be handled.
80 /// Although a `SystemTime` cannot be directly inspected, the [`UNIX_EPOCH`]
81 /// constant is provided in this module as an anchor in time to learn
82 /// information about a `SystemTime`. By calculating the duration from this
83 /// fixed point in time, a `SystemTime` can be converted to a human-readable time,
84 /// or perhaps some other string representation.
86 /// The size of a `SystemTime` struct may vary depending on the target operating
89 /// [`Instant`]: ../../std/time/struct.Instant.html
90 /// [`Result`]: ../../std/result/enum.Result.html
91 /// [`Duration`]: ../../std/time/struct.Duration.html
92 /// [`UNIX_EPOCH`]: ../../std/time/constant.UNIX_EPOCH.html
97 /// use std::time::{Duration, SystemTime};
98 /// use std::thread::sleep;
101 /// let now = SystemTime::now();
103 /// // we sleep for 2 seconds
104 /// sleep(Duration::new(2, 0));
105 /// match now.elapsed() {
108 /// println!("{}", elapsed.as_secs());
111 /// // an error occurred!
112 /// println!("Error: {:?}", e);
117 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
118 #[stable(feature = "time2", since = "1.8.0")]
119 pub struct SystemTime(time::SystemTime);
121 /// An error returned from the `duration_since` and `elapsed` methods on
122 /// `SystemTime`, used to learn how far in the opposite direction a system time
128 /// use std::thread::sleep;
129 /// use std::time::{Duration, SystemTime};
131 /// let sys_time = SystemTime::now();
132 /// sleep(Duration::from_secs(1));
133 /// let new_sys_time = SystemTime::now();
134 /// match sys_time.duration_since(new_sys_time) {
136 /// Err(e) => println!("SystemTimeError difference: {:?}", e.duration()),
139 #[derive(Clone, Debug)]
140 #[stable(feature = "time2", since = "1.8.0")]
141 pub struct SystemTimeError(Duration);
144 /// Returns an instant corresponding to "now".
149 /// use std::time::Instant;
151 /// let now = Instant::now();
153 #[stable(feature = "time2", since = "1.8.0")]
154 pub fn now() -> Instant {
155 let os_now = time::Instant::now();
157 // And here we come upon a sad state of affairs. The whole point of
158 // `Instant` is that it's monotonically increasing. We've found in the
159 // wild, however, that it's not actually monotonically increasing for
160 // one reason or another. These appear to be OS and hardware level bugs,
161 // and there's not really a whole lot we can do about them. Here's a
162 // taste of what we've found:
164 // * #48514 - OpenBSD, x86_64
165 // * #49281 - linux arm64 and s390x
166 // * #51648 - windows, x86
167 // * #56560 - windows, x86_64, AWS
168 // * #56612 - windows, x86, vm (?)
169 // * #56940 - linux, arm64
170 // * https://bugzilla.mozilla.org/show_bug.cgi?id=1487778 - a similar
173 // It simply seems that this it just happens so that a lot in the wild
174 // we're seeing panics across various platforms where consecutive calls
175 // to `Instant::now`, such as via the `elapsed` function, are panicking
176 // as they're going backwards. Placed here is a last-ditch effort to try
177 // to fix things up. We keep a global "latest now" instance which is
178 // returned instead of what the OS says if the OS goes backwards.
180 // To hopefully mitigate the impact of this though a few platforms are
181 // whitelisted as "these at least haven't gone backwards yet".
182 if time::Instant::actually_monotonic() {
183 return Instant(os_now)
186 static LOCK: Mutex = Mutex::new();
187 static mut LAST_NOW: time::Instant = time::Instant::zero();
189 let _lock = LOCK.lock();
190 let now = cmp::max(LAST_NOW, os_now);
196 /// Returns the amount of time elapsed from another instant to this one.
200 /// This function will panic if `earlier` is later than `self`.
205 /// use std::time::{Duration, Instant};
206 /// use std::thread::sleep;
208 /// let now = Instant::now();
209 /// sleep(Duration::new(1, 0));
210 /// let new_now = Instant::now();
211 /// println!("{:?}", new_now.duration_since(now));
213 #[stable(feature = "time2", since = "1.8.0")]
214 pub fn duration_since(&self, earlier: Instant) -> Duration {
215 self.0.sub_instant(&earlier.0)
218 /// Returns the amount of time elapsed from another instant to this one,
219 /// or None if that instant is earlier than this one.
224 /// #![feature(checked_duration_since)]
225 /// use std::time::{Duration, Instant};
226 /// use std::thread::sleep;
228 /// let now = Instant::now();
229 /// sleep(Duration::new(1, 0));
230 /// let new_now = Instant::now();
231 /// println!("{:?}", new_now.checked_duration_since(now));
232 /// println!("{:?}", now.checked_duration_since(new_now)); // None
234 #[unstable(feature = "checked_duration_since", issue = "58402")]
235 pub fn checked_duration_since(&self, earlier: Instant) -> Option<Duration> {
236 if self >= &earlier {
237 Some(self.0.sub_instant(&earlier.0))
243 /// Returns the amount of time elapsed from another instant to this one,
244 /// or zero duration if that instant is earlier than this one.
249 /// #![feature(checked_duration_since)]
250 /// use std::time::{Duration, Instant};
251 /// use std::thread::sleep;
253 /// let now = Instant::now();
254 /// sleep(Duration::new(1, 0));
255 /// let new_now = Instant::now();
256 /// println!("{:?}", new_now.saturating_duration_since(now));
257 /// println!("{:?}", now.saturating_duration_since(new_now)); // 0ns
259 #[unstable(feature = "checked_duration_since", issue = "58402")]
260 pub fn saturating_duration_since(&self, earlier: Instant) -> Duration {
261 self.checked_duration_since(earlier).unwrap_or(Duration::new(0, 0))
264 /// Returns the amount of time elapsed since this instant was created.
268 /// This function may panic if the current time is earlier than this
269 /// instant, which is something that can happen if an `Instant` is
270 /// produced synthetically.
275 /// use std::thread::sleep;
276 /// use std::time::{Duration, Instant};
278 /// let instant = Instant::now();
279 /// let three_secs = Duration::from_secs(3);
280 /// sleep(three_secs);
281 /// assert!(instant.elapsed() >= three_secs);
283 #[stable(feature = "time2", since = "1.8.0")]
284 pub fn elapsed(&self) -> Duration {
285 Instant::now() - *self
288 /// Returns `Some(t)` where `t` is the time `self + duration` if `t` can be represented as
289 /// `Instant` (which means it's inside the bounds of the underlying data structure), `None`
291 #[stable(feature = "time_checked_add", since = "1.34.0")]
292 pub fn checked_add(&self, duration: Duration) -> Option<Instant> {
293 self.0.checked_add_duration(&duration).map(Instant)
296 /// Returns `Some(t)` where `t` is the time `self - duration` if `t` can be represented as
297 /// `Instant` (which means it's inside the bounds of the underlying data structure), `None`
299 #[stable(feature = "time_checked_add", since = "1.34.0")]
300 pub fn checked_sub(&self, duration: Duration) -> Option<Instant> {
301 self.0.checked_sub_duration(&duration).map(Instant)
305 #[stable(feature = "time2", since = "1.8.0")]
306 impl Add<Duration> for Instant {
307 type Output = Instant;
311 /// This function may panic if the resulting point in time cannot be represented by the
312 /// underlying data structure. See [`checked_add`] for a version without panic.
314 /// [`checked_add`]: ../../std/time/struct.Instant.html#method.checked_add
315 fn add(self, other: Duration) -> Instant {
316 self.checked_add(other)
317 .expect("overflow when adding duration to instant")
321 #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
322 impl AddAssign<Duration> for Instant {
323 fn add_assign(&mut self, other: Duration) {
324 *self = *self + other;
328 #[stable(feature = "time2", since = "1.8.0")]
329 impl Sub<Duration> for Instant {
330 type Output = Instant;
332 fn sub(self, other: Duration) -> Instant {
333 self.checked_sub(other)
334 .expect("overflow when subtracting duration from instant")
338 #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
339 impl SubAssign<Duration> for Instant {
340 fn sub_assign(&mut self, other: Duration) {
341 *self = *self - other;
345 #[stable(feature = "time2", since = "1.8.0")]
346 impl Sub<Instant> for Instant {
347 type Output = Duration;
349 fn sub(self, other: Instant) -> Duration {
350 self.duration_since(other)
354 #[stable(feature = "time2", since = "1.8.0")]
355 impl fmt::Debug for Instant {
356 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
362 /// An anchor in time which can be used to create new `SystemTime` instances or
363 /// learn about where in time a `SystemTime` lies.
365 /// This constant is defined to be "1970-01-01 00:00:00 UTC" on all systems with
366 /// respect to the system clock. Using `duration_since` on an existing
367 /// `SystemTime` instance can tell how far away from this point in time a
368 /// measurement lies, and using `UNIX_EPOCH + duration` can be used to create a
369 /// `SystemTime` instance to represent another fixed point in time.
374 /// use std::time::SystemTime;
376 /// match SystemTime::now().duration_since(SystemTime::UNIX_EPOCH) {
377 /// Ok(n) => println!("1970-01-01 00:00:00 UTC was {} seconds ago!", n.as_secs()),
378 /// Err(_) => panic!("SystemTime before UNIX EPOCH!"),
381 #[stable(feature = "assoc_unix_epoch", since = "1.28.0")]
382 pub const UNIX_EPOCH: SystemTime = UNIX_EPOCH;
384 /// Returns the system time corresponding to "now".
389 /// use std::time::SystemTime;
391 /// let sys_time = SystemTime::now();
393 #[stable(feature = "time2", since = "1.8.0")]
394 pub fn now() -> SystemTime {
395 SystemTime(time::SystemTime::now())
398 /// Returns the amount of time elapsed from an earlier point in time.
400 /// This function may fail because measurements taken earlier are not
401 /// guaranteed to always be before later measurements (due to anomalies such
402 /// as the system clock being adjusted either forwards or backwards).
404 /// If successful, [`Ok`]`(`[`Duration`]`)` is returned where the duration represents
405 /// the amount of time elapsed from the specified measurement to this one.
407 /// Returns an [`Err`] if `earlier` is later than `self`, and the error
408 /// contains how far from `self` the time is.
410 /// [`Ok`]: ../../std/result/enum.Result.html#variant.Ok
411 /// [`Duration`]: ../../std/time/struct.Duration.html
412 /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
417 /// use std::time::SystemTime;
419 /// let sys_time = SystemTime::now();
420 /// let difference = sys_time.duration_since(sys_time)
421 /// .expect("SystemTime::duration_since failed");
422 /// println!("{:?}", difference);
424 #[stable(feature = "time2", since = "1.8.0")]
425 pub fn duration_since(&self, earlier: SystemTime)
426 -> Result<Duration, SystemTimeError> {
427 self.0.sub_time(&earlier.0).map_err(SystemTimeError)
430 /// Returns the amount of time elapsed since this system time was created.
432 /// This function may fail as the underlying system clock is susceptible to
433 /// drift and updates (e.g., the system clock could go backwards), so this
434 /// function may not always succeed. If successful, [`Ok`]`(`[`Duration`]`)` is
435 /// returned where the duration represents the amount of time elapsed from
436 /// this time measurement to the current time.
438 /// Returns an [`Err`] if `self` is later than the current system time, and
439 /// the error contains how far from the current system time `self` is.
441 /// [`Ok`]: ../../std/result/enum.Result.html#variant.Ok
442 /// [`Duration`]: ../../std/time/struct.Duration.html
443 /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
448 /// use std::thread::sleep;
449 /// use std::time::{Duration, SystemTime};
451 /// let sys_time = SystemTime::now();
452 /// let one_sec = Duration::from_secs(1);
454 /// assert!(sys_time.elapsed().unwrap() >= one_sec);
456 #[stable(feature = "time2", since = "1.8.0")]
457 pub fn elapsed(&self) -> Result<Duration, SystemTimeError> {
458 SystemTime::now().duration_since(*self)
461 /// Returns `Some(t)` where `t` is the time `self + duration` if `t` can be represented as
462 /// `SystemTime` (which means it's inside the bounds of the underlying data structure), `None`
464 #[stable(feature = "time_checked_add", since = "1.34.0")]
465 pub fn checked_add(&self, duration: Duration) -> Option<SystemTime> {
466 self.0.checked_add_duration(&duration).map(SystemTime)
469 /// Returns `Some(t)` where `t` is the time `self - duration` if `t` can be represented as
470 /// `SystemTime` (which means it's inside the bounds of the underlying data structure), `None`
472 #[stable(feature = "time_checked_add", since = "1.34.0")]
473 pub fn checked_sub(&self, duration: Duration) -> Option<SystemTime> {
474 self.0.checked_sub_duration(&duration).map(SystemTime)
478 #[stable(feature = "time2", since = "1.8.0")]
479 impl Add<Duration> for SystemTime {
480 type Output = SystemTime;
484 /// This function may panic if the resulting point in time cannot be represented by the
485 /// underlying data structure. See [`checked_add`] for a version without panic.
487 /// [`checked_add`]: ../../std/time/struct.SystemTime.html#method.checked_add
488 fn add(self, dur: Duration) -> SystemTime {
489 self.checked_add(dur)
490 .expect("overflow when adding duration to instant")
494 #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
495 impl AddAssign<Duration> for SystemTime {
496 fn add_assign(&mut self, other: Duration) {
497 *self = *self + other;
501 #[stable(feature = "time2", since = "1.8.0")]
502 impl Sub<Duration> for SystemTime {
503 type Output = SystemTime;
505 fn sub(self, dur: Duration) -> SystemTime {
506 self.checked_sub(dur)
507 .expect("overflow when subtracting duration from instant")
511 #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
512 impl SubAssign<Duration> for SystemTime {
513 fn sub_assign(&mut self, other: Duration) {
514 *self = *self - other;
518 #[stable(feature = "time2", since = "1.8.0")]
519 impl fmt::Debug for SystemTime {
520 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
525 /// An anchor in time which can be used to create new `SystemTime` instances or
526 /// learn about where in time a `SystemTime` lies.
528 /// This constant is defined to be "1970-01-01 00:00:00 UTC" on all systems with
529 /// respect to the system clock. Using `duration_since` on an existing
530 /// [`SystemTime`] instance can tell how far away from this point in time a
531 /// measurement lies, and using `UNIX_EPOCH + duration` can be used to create a
532 /// [`SystemTime`] instance to represent another fixed point in time.
534 /// [`SystemTime`]: ../../std/time/struct.SystemTime.html
539 /// use std::time::{SystemTime, UNIX_EPOCH};
541 /// match SystemTime::now().duration_since(UNIX_EPOCH) {
542 /// Ok(n) => println!("1970-01-01 00:00:00 UTC was {} seconds ago!", n.as_secs()),
543 /// Err(_) => panic!("SystemTime before UNIX EPOCH!"),
546 #[stable(feature = "time2", since = "1.8.0")]
547 pub const UNIX_EPOCH: SystemTime = SystemTime(time::UNIX_EPOCH);
549 impl SystemTimeError {
550 /// Returns the positive duration which represents how far forward the
551 /// second system time was from the first.
553 /// A `SystemTimeError` is returned from the [`duration_since`] and [`elapsed`]
554 /// methods of [`SystemTime`] whenever the second system time represents a point later
555 /// in time than the `self` of the method call.
557 /// [`duration_since`]: ../../std/time/struct.SystemTime.html#method.duration_since
558 /// [`elapsed`]: ../../std/time/struct.SystemTime.html#method.elapsed
559 /// [`SystemTime`]: ../../std/time/struct.SystemTime.html
564 /// use std::thread::sleep;
565 /// use std::time::{Duration, SystemTime};
567 /// let sys_time = SystemTime::now();
568 /// sleep(Duration::from_secs(1));
569 /// let new_sys_time = SystemTime::now();
570 /// match sys_time.duration_since(new_sys_time) {
572 /// Err(e) => println!("SystemTimeError difference: {:?}", e.duration()),
575 #[stable(feature = "time2", since = "1.8.0")]
576 pub fn duration(&self) -> Duration {
581 #[stable(feature = "time2", since = "1.8.0")]
582 impl Error for SystemTimeError {
583 fn description(&self) -> &str { "other time was not earlier than self" }
586 #[stable(feature = "time2", since = "1.8.0")]
587 impl fmt::Display for SystemTimeError {
588 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
589 write!(f, "second time provided was later than self")
593 impl FromInner<time::SystemTime> for SystemTime {
594 fn from_inner(time: time::SystemTime) -> SystemTime {
601 use super::{Instant, SystemTime, Duration, UNIX_EPOCH};
603 macro_rules! assert_almost_eq {
604 ($a:expr, $b:expr) => ({
605 let (a, b) = ($a, $b);
607 let (a, b) = if a > b {(a, b)} else {(b, a)};
608 assert!(a - Duration::new(0, 1000) <= b,
609 "{:?} is not almost equal to {:?}", a, b);
615 fn instant_monotonic() {
616 let a = Instant::now();
617 let b = Instant::now();
622 fn instant_elapsed() {
623 let a = Instant::now();
629 let a = Instant::now();
630 let b = Instant::now();
631 println!("a: {:?}", a);
632 println!("b: {:?}", b);
633 let dur = b.duration_since(a);
634 println!("dur: {:?}", dur);
635 assert_almost_eq!(b - dur, a);
636 assert_almost_eq!(a + dur, b);
638 let second = Duration::new(1, 0);
639 assert_almost_eq!(a - second + second, a);
640 assert_almost_eq!(a.checked_sub(second).unwrap().checked_add(second).unwrap(), a);
642 // checked_add_duration will not panic on overflow
643 let mut maybe_t = Some(Instant::now());
644 let max_duration = Duration::from_secs(u64::max_value());
645 // in case `Instant` can store `>= now + max_duration`.
647 maybe_t = maybe_t.and_then(|t| t.checked_add(max_duration));
649 assert_eq!(maybe_t, None);
651 // checked_add_duration calculates the right time and will work for another year
652 let year = Duration::from_secs(60 * 60 * 24 * 365);
653 assert_eq!(a + year, a.checked_add(year).unwrap());
657 fn instant_math_is_associative() {
658 let now = Instant::now();
659 let offset = Duration::from_millis(5);
660 // Changing the order of instant math shouldn't change the results,
661 // especially when the expression reduces to X + identity.
662 assert_eq!((now + offset) - now, (now - now) + offset);
667 fn instant_duration_panic() {
668 let a = Instant::now();
669 (a - Duration::new(1, 0)).duration_since(a);
673 fn checked_instant_duration_nopanic() {
674 let a = Instant::now();
675 let ret = (a - Duration::new(1, 0)).checked_duration_since(a);
676 assert_eq!(ret, None);
680 fn saturating_instant_duration_nopanic() {
681 let a = Instant::now();
682 let ret = (a - Duration::new(1, 0)).saturating_duration_since(a);
683 assert_eq!(ret, Duration::new(0,0));
687 fn system_time_math() {
688 let a = SystemTime::now();
689 let b = SystemTime::now();
690 match b.duration_since(a) {
691 Ok(dur) if dur == Duration::new(0, 0) => {
692 assert_almost_eq!(a, b);
696 assert_almost_eq!(b - dur, a);
697 assert_almost_eq!(a + dur, b);
700 let dur = dur.duration();
702 assert_almost_eq!(b + dur, a);
703 assert_almost_eq!(a - dur, b);
707 let second = Duration::new(1, 0);
708 assert_almost_eq!(a.duration_since(a - second).unwrap(), second);
709 assert_almost_eq!(a.duration_since(a + second).unwrap_err()
710 .duration(), second);
712 assert_almost_eq!(a - second + second, a);
713 assert_almost_eq!(a.checked_sub(second).unwrap().checked_add(second).unwrap(), a);
715 let one_second_from_epoch = UNIX_EPOCH + Duration::new(1, 0);
716 let one_second_from_epoch2 = UNIX_EPOCH + Duration::new(0, 500_000_000)
717 + Duration::new(0, 500_000_000);
718 assert_eq!(one_second_from_epoch, one_second_from_epoch2);
720 // checked_add_duration will not panic on overflow
721 let mut maybe_t = Some(SystemTime::UNIX_EPOCH);
722 let max_duration = Duration::from_secs(u64::max_value());
723 // in case `SystemTime` can store `>= UNIX_EPOCH + max_duration`.
725 maybe_t = maybe_t.and_then(|t| t.checked_add(max_duration));
727 assert_eq!(maybe_t, None);
729 // checked_add_duration calculates the right time and will work for another year
730 let year = Duration::from_secs(60 * 60 * 24 * 365);
731 assert_eq!(a + year, a.checked_add(year).unwrap());
735 fn system_time_elapsed() {
736 let a = SystemTime::now();
742 let ts = SystemTime::now();
743 let a = ts.duration_since(UNIX_EPOCH + Duration::new(1, 0)).unwrap();
744 let b = ts.duration_since(UNIX_EPOCH).unwrap();
746 assert_eq!(b - a, Duration::new(1, 0));
748 let thirty_years = Duration::new(1, 0) * 60 * 60 * 24 * 365 * 30;
750 // Right now for CI this test is run in an emulator, and apparently the
751 // aarch64 emulator's sense of time is that we're still living in the
754 // Otherwise let's assume that we're all running computers later than
756 if !cfg!(target_arch = "aarch64") {
757 assert!(a > thirty_years);
760 // let's assume that we're all running computers earlier than 2090.
761 // Should give us ~70 years to fix this!
762 let hundred_twenty_years = thirty_years * 4;
763 assert!(a < hundred_twenty_years);