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, AddAssign, Sub, SubAssign};
20 use crate::sys_common::mutex::Mutex;
21 use crate::sys_common::FromInner;
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());
63 /// # OS-specific behaviors
65 /// An `Instant` is a wrapper around system-specific types and it may behave
66 /// differently depending on the underlying operating system. For example,
67 /// the following snippet is fine on Linux but panics on macOS:
70 /// use std::time::{Instant, Duration};
72 /// let now = Instant::now();
73 /// let max_nanoseconds = u64::MAX / 1_000_000_000;
74 /// let duration = Duration::new(max_nanoseconds, 0);
75 /// println!("{:?}", now + duration);
78 /// # Underlying System calls
79 /// Currently, the following system calls are being used to get the current time using `now()`:
81 /// | Platform | System call |
82 /// |:---------:|:--------------------------------------------------------------------:|
83 /// | CloudABI | [clock_time_get (Monotonic Clock)] |
84 /// | SGX | [`insecure_time` usercall]. More information on [timekeeping in SGX] |
85 /// | UNIX | [clock_gettime (Monotonic Clock)] |
86 /// | Darwin | [mach_absolute_time] |
87 /// | VXWorks | [clock_gettime (Monotonic Clock)] |
88 /// | WASI | [__wasi_clock_time_get (Monotonic Clock)] |
89 /// | Windows | [QueryPerformanceCounter] |
91 /// [QueryPerformanceCounter]: https://docs.microsoft.com/en-us/windows/win32/api/profileapi/nf-profileapi-queryperformancecounter
92 /// [`insecure_time` usercall]: https://edp.fortanix.com/docs/api/fortanix_sgx_abi/struct.Usercalls.html#method.insecure_time
93 /// [timekeeping in SGX]: https://edp.fortanix.com/docs/concepts/rust-std/#codestdtimecode
94 /// [__wasi_clock_time_get (Monotonic Clock)]: https://github.com/WebAssembly/WASI/blob/master/phases/snapshot/docs.md#clock_time_get
95 /// [clock_gettime (Monotonic Clock)]: https://linux.die.net/man/3/clock_gettime
96 /// [mach_absolute_time]: https://developer.apple.com/library/archive/documentation/Darwin/Conceptual/KernelProgramming/services/services.html
97 /// [clock_time_get (Monotonic Clock)]: https://nuxi.nl/cloudabi/#clock_time_get
99 /// **Disclaimer:** These system calls might change over time.
100 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
101 #[stable(feature = "time2", since = "1.8.0")]
102 pub struct Instant(time::Instant);
104 /// A measurement of the system clock, useful for talking to
105 /// external entities like the file system or other processes.
107 /// Distinct from the [`Instant`] type, this time measurement **is not
108 /// monotonic**. This means that you can save a file to the file system, then
109 /// save another file to the file system, **and the second file has a
110 /// `SystemTime` measurement earlier than the first**. In other words, an
111 /// operation that happens after another operation in real time may have an
112 /// earlier `SystemTime`!
114 /// Consequently, comparing two `SystemTime` instances to learn about the
115 /// duration between them returns a [`Result`] instead of an infallible [`Duration`]
116 /// to indicate that this sort of time drift may happen and needs to be handled.
118 /// Although a `SystemTime` cannot be directly inspected, the [`UNIX_EPOCH`]
119 /// constant is provided in this module as an anchor in time to learn
120 /// information about a `SystemTime`. By calculating the duration from this
121 /// fixed point in time, a `SystemTime` can be converted to a human-readable time,
122 /// or perhaps some other string representation.
124 /// The size of a `SystemTime` struct may vary depending on the target operating
130 /// use std::time::{Duration, SystemTime};
131 /// use std::thread::sleep;
134 /// let now = SystemTime::now();
136 /// // we sleep for 2 seconds
137 /// sleep(Duration::new(2, 0));
138 /// match now.elapsed() {
141 /// println!("{}", elapsed.as_secs());
144 /// // an error occurred!
145 /// println!("Error: {:?}", e);
151 /// # Underlying System calls
152 /// Currently, the following system calls are being used to get the current time using `now()`:
154 /// | Platform | System call |
155 /// |:---------:|:--------------------------------------------------------------------:|
156 /// | CloudABI | [clock_time_get (Realtime Clock)] |
157 /// | SGX | [`insecure_time` usercall]. More information on [timekeeping in SGX] |
158 /// | UNIX | [clock_gettime (Realtime Clock)] |
159 /// | DARWIN | [gettimeofday] |
160 /// | VXWorks | [clock_gettime (Realtime Clock)] |
161 /// | WASI | [__wasi_clock_time_get (Realtime Clock)] |
162 /// | Windows | [GetSystemTimePreciseAsFileTime] / [GetSystemTimeAsFileTime] |
164 /// [clock_time_get (Realtime Clock)]: https://nuxi.nl/cloudabi/#clock_time_get
165 /// [`insecure_time` usercall]: https://edp.fortanix.com/docs/api/fortanix_sgx_abi/struct.Usercalls.html#method.insecure_time
166 /// [timekeeping in SGX]: https://edp.fortanix.com/docs/concepts/rust-std/#codestdtimecode
167 /// [gettimeofday]: http://man7.org/linux/man-pages/man2/gettimeofday.2.html
168 /// [clock_gettime (Realtime Clock)]: https://linux.die.net/man/3/clock_gettime
169 /// [__wasi_clock_time_get (Realtime Clock)]: https://github.com/WebAssembly/WASI/blob/master/phases/snapshot/docs.md#clock_time_get
170 /// [GetSystemTimePreciseAsFileTime]: https://docs.microsoft.com/en-us/windows/win32/api/sysinfoapi/nf-sysinfoapi-getsystemtimepreciseasfiletime
171 /// [GetSystemTimeAsFileTime]: https://docs.microsoft.com/en-us/windows/win32/api/sysinfoapi/nf-sysinfoapi-getsystemtimeasfiletime
173 /// **Disclaimer:** These system calls might change over time.
174 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
175 #[stable(feature = "time2", since = "1.8.0")]
176 pub struct SystemTime(time::SystemTime);
178 /// An error returned from the `duration_since` and `elapsed` methods on
179 /// `SystemTime`, used to learn how far in the opposite direction a system time
185 /// use std::thread::sleep;
186 /// use std::time::{Duration, SystemTime};
188 /// let sys_time = SystemTime::now();
189 /// sleep(Duration::from_secs(1));
190 /// let new_sys_time = SystemTime::now();
191 /// match sys_time.duration_since(new_sys_time) {
193 /// Err(e) => println!("SystemTimeError difference: {:?}", e.duration()),
196 #[derive(Clone, Debug)]
197 #[stable(feature = "time2", since = "1.8.0")]
198 pub struct SystemTimeError(Duration);
201 /// Returns an instant corresponding to "now".
206 /// use std::time::Instant;
208 /// let now = Instant::now();
210 #[stable(feature = "time2", since = "1.8.0")]
211 pub fn now() -> Instant {
212 let os_now = time::Instant::now();
214 // And here we come upon a sad state of affairs. The whole point of
215 // `Instant` is that it's monotonically increasing. We've found in the
216 // wild, however, that it's not actually monotonically increasing for
217 // one reason or another. These appear to be OS and hardware level bugs,
218 // and there's not really a whole lot we can do about them. Here's a
219 // taste of what we've found:
221 // * #48514 - OpenBSD, x86_64
222 // * #49281 - linux arm64 and s390x
223 // * #51648 - windows, x86
224 // * #56560 - windows, x86_64, AWS
225 // * #56612 - windows, x86, vm (?)
226 // * #56940 - linux, arm64
227 // * https://bugzilla.mozilla.org/show_bug.cgi?id=1487778 - a similar
230 // It seems that this just happens a lot in the wild.
231 // We're seeing panics across various platforms where consecutive calls
232 // to `Instant::now`, such as via the `elapsed` function, are panicking
233 // as they're going backwards. Placed here is a last-ditch effort to try
234 // to fix things up. We keep a global "latest now" instance which is
235 // returned instead of what the OS says if the OS goes backwards.
237 // To hopefully mitigate the impact of this, a few platforms are
238 // excluded as "these at least haven't gone backwards yet".
239 if time::Instant::actually_monotonic() {
240 return Instant(os_now);
243 static LOCK: Mutex = Mutex::new();
244 static mut LAST_NOW: time::Instant = time::Instant::zero();
246 let _lock = LOCK.lock();
247 let now = cmp::max(LAST_NOW, os_now);
253 /// Returns the amount of time elapsed from another instant to this one.
257 /// This function will panic if `earlier` is later than `self`.
262 /// use std::time::{Duration, Instant};
263 /// use std::thread::sleep;
265 /// let now = Instant::now();
266 /// sleep(Duration::new(1, 0));
267 /// let new_now = Instant::now();
268 /// println!("{:?}", new_now.duration_since(now));
270 #[stable(feature = "time2", since = "1.8.0")]
271 pub fn duration_since(&self, earlier: Instant) -> Duration {
272 self.0.checked_sub_instant(&earlier.0).expect("supplied instant is later than self")
275 /// Returns the amount of time elapsed from another instant to this one,
276 /// or None if that instant is later than this one.
281 /// use std::time::{Duration, Instant};
282 /// use std::thread::sleep;
284 /// let now = Instant::now();
285 /// sleep(Duration::new(1, 0));
286 /// let new_now = Instant::now();
287 /// println!("{:?}", new_now.checked_duration_since(now));
288 /// println!("{:?}", now.checked_duration_since(new_now)); // None
290 #[stable(feature = "checked_duration_since", since = "1.39.0")]
291 pub fn checked_duration_since(&self, earlier: Instant) -> Option<Duration> {
292 self.0.checked_sub_instant(&earlier.0)
295 /// Returns the amount of time elapsed from another instant to this one,
296 /// or zero duration if that instant is later than this one.
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.saturating_duration_since(now));
308 /// println!("{:?}", now.saturating_duration_since(new_now)); // 0ns
310 #[stable(feature = "checked_duration_since", since = "1.39.0")]
311 pub fn saturating_duration_since(&self, earlier: Instant) -> Duration {
312 self.checked_duration_since(earlier).unwrap_or(Duration::new(0, 0))
315 /// Returns the amount of time elapsed since this instant was created.
319 /// This function may panic if the current time is earlier than this
320 /// instant, which is something that can happen if an `Instant` is
321 /// produced synthetically.
326 /// use std::thread::sleep;
327 /// use std::time::{Duration, Instant};
329 /// let instant = Instant::now();
330 /// let three_secs = Duration::from_secs(3);
331 /// sleep(three_secs);
332 /// assert!(instant.elapsed() >= three_secs);
334 #[stable(feature = "time2", since = "1.8.0")]
335 pub fn elapsed(&self) -> Duration {
336 Instant::now() - *self
339 /// Returns `Some(t)` where `t` is the time `self + duration` if `t` can be represented as
340 /// `Instant` (which means it's inside the bounds of the underlying data structure), `None`
342 #[stable(feature = "time_checked_add", since = "1.34.0")]
343 pub fn checked_add(&self, duration: Duration) -> Option<Instant> {
344 self.0.checked_add_duration(&duration).map(Instant)
347 /// Returns `Some(t)` where `t` is the time `self - duration` if `t` can be represented as
348 /// `Instant` (which means it's inside the bounds of the underlying data structure), `None`
350 #[stable(feature = "time_checked_add", since = "1.34.0")]
351 pub fn checked_sub(&self, duration: Duration) -> Option<Instant> {
352 self.0.checked_sub_duration(&duration).map(Instant)
356 #[stable(feature = "time2", since = "1.8.0")]
357 impl Add<Duration> for Instant {
358 type Output = Instant;
362 /// This function may panic if the resulting point in time cannot be represented by the
363 /// underlying data structure. See [`Instant::checked_add`] for a version without panic.
364 fn add(self, other: Duration) -> Instant {
365 self.checked_add(other).expect("overflow when adding duration to instant")
369 #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
370 impl AddAssign<Duration> for Instant {
371 fn add_assign(&mut self, other: Duration) {
372 *self = *self + other;
376 #[stable(feature = "time2", since = "1.8.0")]
377 impl Sub<Duration> for Instant {
378 type Output = Instant;
380 fn sub(self, other: Duration) -> Instant {
381 self.checked_sub(other).expect("overflow when subtracting duration from instant")
385 #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
386 impl SubAssign<Duration> for Instant {
387 fn sub_assign(&mut self, other: Duration) {
388 *self = *self - other;
392 #[stable(feature = "time2", since = "1.8.0")]
393 impl Sub<Instant> for Instant {
394 type Output = Duration;
396 fn sub(self, other: Instant) -> Duration {
397 self.duration_since(other)
401 #[stable(feature = "time2", since = "1.8.0")]
402 impl fmt::Debug for Instant {
403 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
409 /// An anchor in time which can be used to create new `SystemTime` instances or
410 /// learn about where in time a `SystemTime` lies.
412 /// This constant is defined to be "1970-01-01 00:00:00 UTC" on all systems with
413 /// respect to the system clock. Using `duration_since` on an existing
414 /// `SystemTime` instance can tell how far away from this point in time a
415 /// measurement lies, and using `UNIX_EPOCH + duration` can be used to create a
416 /// `SystemTime` instance to represent another fixed point in time.
421 /// use std::time::SystemTime;
423 /// match SystemTime::now().duration_since(SystemTime::UNIX_EPOCH) {
424 /// Ok(n) => println!("1970-01-01 00:00:00 UTC was {} seconds ago!", n.as_secs()),
425 /// Err(_) => panic!("SystemTime before UNIX EPOCH!"),
428 #[stable(feature = "assoc_unix_epoch", since = "1.28.0")]
429 pub const UNIX_EPOCH: SystemTime = UNIX_EPOCH;
431 /// Returns the system time corresponding to "now".
436 /// use std::time::SystemTime;
438 /// let sys_time = SystemTime::now();
440 #[stable(feature = "time2", since = "1.8.0")]
441 pub fn now() -> SystemTime {
442 SystemTime(time::SystemTime::now())
445 /// Returns the amount of time elapsed from an earlier point in time.
447 /// This function may fail because measurements taken earlier are not
448 /// guaranteed to always be before later measurements (due to anomalies such
449 /// as the system clock being adjusted either forwards or backwards).
450 /// [`Instant`] can be used to measure elapsed time without this risk of failure.
452 /// If successful, [`Ok`]`(`[`Duration`]`)` is returned where the duration represents
453 /// the amount of time elapsed from the specified measurement to this one.
455 /// Returns an [`Err`] if `earlier` is later than `self`, and the error
456 /// contains how far from `self` the time is.
461 /// use std::time::SystemTime;
463 /// let sys_time = SystemTime::now();
464 /// let new_sys_time = SystemTime::now();
465 /// let difference = new_sys_time.duration_since(sys_time)
466 /// .expect("Clock may have gone backwards");
467 /// println!("{:?}", difference);
469 #[stable(feature = "time2", since = "1.8.0")]
470 pub fn duration_since(&self, earlier: SystemTime) -> Result<Duration, SystemTimeError> {
471 self.0.sub_time(&earlier.0).map_err(SystemTimeError)
474 /// Returns the difference between the clock time when this
475 /// system time was created, and the current clock time.
477 /// This function may fail as the underlying system clock is susceptible to
478 /// drift and updates (e.g., the system clock could go backwards), so this
479 /// function may not always succeed. If successful, [`Ok`]`(`[`Duration`]`)` is
480 /// returned where the duration represents the amount of time elapsed from
481 /// this time measurement to the current time.
483 /// To measure elapsed time reliably, use [`Instant`] instead.
485 /// Returns an [`Err`] if `self` is later than the current system time, and
486 /// the error contains how far from the current system time `self` is.
491 /// use std::thread::sleep;
492 /// use std::time::{Duration, SystemTime};
494 /// let sys_time = SystemTime::now();
495 /// let one_sec = Duration::from_secs(1);
497 /// assert!(sys_time.elapsed().unwrap() >= one_sec);
499 #[stable(feature = "time2", since = "1.8.0")]
500 pub fn elapsed(&self) -> Result<Duration, SystemTimeError> {
501 SystemTime::now().duration_since(*self)
504 /// Returns `Some(t)` where `t` is the time `self + duration` if `t` can be represented as
505 /// `SystemTime` (which means it's inside the bounds of the underlying data structure), `None`
507 #[stable(feature = "time_checked_add", since = "1.34.0")]
508 pub fn checked_add(&self, duration: Duration) -> Option<SystemTime> {
509 self.0.checked_add_duration(&duration).map(SystemTime)
512 /// Returns `Some(t)` where `t` is the time `self - duration` if `t` can be represented as
513 /// `SystemTime` (which means it's inside the bounds of the underlying data structure), `None`
515 #[stable(feature = "time_checked_add", since = "1.34.0")]
516 pub fn checked_sub(&self, duration: Duration) -> Option<SystemTime> {
517 self.0.checked_sub_duration(&duration).map(SystemTime)
521 #[stable(feature = "time2", since = "1.8.0")]
522 impl Add<Duration> for SystemTime {
523 type Output = SystemTime;
527 /// This function may panic if the resulting point in time cannot be represented by the
528 /// underlying data structure. See [`SystemTime::checked_add`] for a version without panic.
529 fn add(self, dur: Duration) -> SystemTime {
530 self.checked_add(dur).expect("overflow when adding duration to instant")
534 #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
535 impl AddAssign<Duration> for SystemTime {
536 fn add_assign(&mut self, other: Duration) {
537 *self = *self + other;
541 #[stable(feature = "time2", since = "1.8.0")]
542 impl Sub<Duration> for SystemTime {
543 type Output = SystemTime;
545 fn sub(self, dur: Duration) -> SystemTime {
546 self.checked_sub(dur).expect("overflow when subtracting duration from instant")
550 #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
551 impl SubAssign<Duration> for SystemTime {
552 fn sub_assign(&mut self, other: Duration) {
553 *self = *self - other;
557 #[stable(feature = "time2", since = "1.8.0")]
558 impl fmt::Debug for SystemTime {
559 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
564 /// An anchor in time which can be used to create new `SystemTime` instances or
565 /// learn about where in time a `SystemTime` lies.
567 /// This constant is defined to be "1970-01-01 00:00:00 UTC" on all systems with
568 /// respect to the system clock. Using `duration_since` on an existing
569 /// [`SystemTime`] instance can tell how far away from this point in time a
570 /// measurement lies, and using `UNIX_EPOCH + duration` can be used to create a
571 /// [`SystemTime`] instance to represent another fixed point in time.
576 /// use std::time::{SystemTime, UNIX_EPOCH};
578 /// match SystemTime::now().duration_since(UNIX_EPOCH) {
579 /// Ok(n) => println!("1970-01-01 00:00:00 UTC was {} seconds ago!", n.as_secs()),
580 /// Err(_) => panic!("SystemTime before UNIX EPOCH!"),
583 #[stable(feature = "time2", since = "1.8.0")]
584 pub const UNIX_EPOCH: SystemTime = SystemTime(time::UNIX_EPOCH);
586 impl SystemTimeError {
587 /// Returns the positive duration which represents how far forward the
588 /// second system time was from the first.
590 /// A `SystemTimeError` is returned from the [`SystemTime::duration_since`]
591 /// and [`SystemTime::elapsed`] methods whenever the second system time
592 /// represents a point later in time than the `self` of the method call.
597 /// use std::thread::sleep;
598 /// use std::time::{Duration, SystemTime};
600 /// let sys_time = SystemTime::now();
601 /// sleep(Duration::from_secs(1));
602 /// let new_sys_time = SystemTime::now();
603 /// match sys_time.duration_since(new_sys_time) {
605 /// Err(e) => println!("SystemTimeError difference: {:?}", e.duration()),
608 #[stable(feature = "time2", since = "1.8.0")]
609 pub fn duration(&self) -> Duration {
614 #[stable(feature = "time2", since = "1.8.0")]
615 impl Error for SystemTimeError {
617 fn description(&self) -> &str {
618 "other time was not earlier than self"
622 #[stable(feature = "time2", since = "1.8.0")]
623 impl fmt::Display for SystemTimeError {
624 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
625 write!(f, "second time provided was later than self")
629 impl FromInner<time::SystemTime> for SystemTime {
630 fn from_inner(time: time::SystemTime) -> SystemTime {
637 use super::{Duration, Instant, SystemTime, UNIX_EPOCH};
639 macro_rules! assert_almost_eq {
640 ($a:expr, $b:expr) => {{
641 let (a, b) = ($a, $b);
643 let (a, b) = if a > b { (a, b) } else { (b, a) };
644 assert!(a - Duration::new(0, 1000) <= b, "{:?} is not almost equal to {:?}", a, b);
650 fn instant_monotonic() {
651 let a = Instant::now();
652 let b = Instant::now();
657 fn instant_elapsed() {
658 let a = Instant::now();
664 let a = Instant::now();
665 let b = Instant::now();
666 println!("a: {:?}", a);
667 println!("b: {:?}", b);
668 let dur = b.duration_since(a);
669 println!("dur: {:?}", dur);
670 assert_almost_eq!(b - dur, a);
671 assert_almost_eq!(a + dur, b);
673 let second = Duration::new(1, 0);
674 assert_almost_eq!(a - second + second, a);
675 assert_almost_eq!(a.checked_sub(second).unwrap().checked_add(second).unwrap(), a);
677 // checked_add_duration will not panic on overflow
678 let mut maybe_t = Some(Instant::now());
679 let max_duration = Duration::from_secs(u64::MAX);
680 // in case `Instant` can store `>= now + max_duration`.
682 maybe_t = maybe_t.and_then(|t| t.checked_add(max_duration));
684 assert_eq!(maybe_t, None);
686 // checked_add_duration calculates the right time and will work for another year
687 let year = Duration::from_secs(60 * 60 * 24 * 365);
688 assert_eq!(a + year, a.checked_add(year).unwrap());
692 fn instant_math_is_associative() {
693 let now = Instant::now();
694 let offset = Duration::from_millis(5);
695 // Changing the order of instant math shouldn't change the results,
696 // especially when the expression reduces to X + identity.
697 assert_eq!((now + offset) - now, (now - now) + offset);
702 fn instant_duration_since_panic() {
703 let a = Instant::now();
704 (a - Duration::new(1, 0)).duration_since(a);
708 fn instant_checked_duration_since_nopanic() {
709 let now = Instant::now();
710 let earlier = now - Duration::new(1, 0);
711 let later = now + Duration::new(1, 0);
712 assert_eq!(earlier.checked_duration_since(now), None);
713 assert_eq!(later.checked_duration_since(now), Some(Duration::new(1, 0)));
714 assert_eq!(now.checked_duration_since(now), Some(Duration::new(0, 0)));
718 fn instant_saturating_duration_since_nopanic() {
719 let a = Instant::now();
720 let ret = (a - Duration::new(1, 0)).saturating_duration_since(a);
721 assert_eq!(ret, Duration::new(0, 0));
725 fn system_time_math() {
726 let a = SystemTime::now();
727 let b = SystemTime::now();
728 match b.duration_since(a) {
729 Ok(dur) if dur == Duration::new(0, 0) => {
730 assert_almost_eq!(a, b);
734 assert_almost_eq!(b - dur, a);
735 assert_almost_eq!(a + dur, b);
738 let dur = dur.duration();
740 assert_almost_eq!(b + dur, a);
741 assert_almost_eq!(a - dur, b);
745 let second = Duration::new(1, 0);
746 assert_almost_eq!(a.duration_since(a - second).unwrap(), second);
747 assert_almost_eq!(a.duration_since(a + second).unwrap_err().duration(), second);
749 assert_almost_eq!(a - second + second, a);
750 assert_almost_eq!(a.checked_sub(second).unwrap().checked_add(second).unwrap(), a);
752 let one_second_from_epoch = UNIX_EPOCH + Duration::new(1, 0);
753 let one_second_from_epoch2 =
754 UNIX_EPOCH + Duration::new(0, 500_000_000) + Duration::new(0, 500_000_000);
755 assert_eq!(one_second_from_epoch, one_second_from_epoch2);
757 // checked_add_duration will not panic on overflow
758 let mut maybe_t = Some(SystemTime::UNIX_EPOCH);
759 let max_duration = Duration::from_secs(u64::MAX);
760 // in case `SystemTime` can store `>= UNIX_EPOCH + max_duration`.
762 maybe_t = maybe_t.and_then(|t| t.checked_add(max_duration));
764 assert_eq!(maybe_t, None);
766 // checked_add_duration calculates the right time and will work for another year
767 let year = Duration::from_secs(60 * 60 * 24 * 365);
768 assert_eq!(a + year, a.checked_add(year).unwrap());
772 fn system_time_elapsed() {
773 let a = SystemTime::now();
779 let ts = SystemTime::now();
780 let a = ts.duration_since(UNIX_EPOCH + Duration::new(1, 0)).unwrap();
781 let b = ts.duration_since(UNIX_EPOCH).unwrap();
783 assert_eq!(b - a, Duration::new(1, 0));
785 let thirty_years = Duration::new(1, 0) * 60 * 60 * 24 * 365 * 30;
787 // Right now for CI this test is run in an emulator, and apparently the
788 // aarch64 emulator's sense of time is that we're still living in the
789 // 70s. This is also true for riscv (also qemu)
791 // Otherwise let's assume that we're all running computers later than
793 if !cfg!(target_arch = "aarch64") && !cfg!(target_arch = "riscv64") {
794 assert!(a > thirty_years);
797 // let's assume that we're all running computers earlier than 2090.
798 // Should give us ~70 years to fix this!
799 let hundred_twenty_years = thirty_years * 4;
800 assert!(a < hundred_twenty_years);