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Document that SystemTime does not count leap seconds #109660

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Merged
merged 3 commits into from
Aug 28, 2023
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Document that SystemTime does not count leap seconds #109660

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f2649cb
Add note about clone-and-hack to the two UNIX_EPOCH definitions
ijackson Mar 27, 2023
e329b23
Document that SystemTime does not count leap seconds
ijackson Mar 27, 2023
c4bc16c
Adjust the duration_since(UNIX_EPOCH) docs
ijackson Aug 7, 2023
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26 changes: 26 additions & 0 deletions library/std/src/time.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -176,6 +176,14 @@ pub struct Instant(time::Instant);
/// The size of a `SystemTime` struct may vary depending on the target operating
/// system.
///
/// A `SystemTime` does not count leap seconds.
/// `SystemTime::now()`'s behaviour around a leap second
/// is the same as the operating system's wall clock.
/// The precise behaviour near a leap second
/// (e.g. whether the clock appears to run slow or fast, or stop, or jump)
/// depends on platform and configuration,
/// so should not be relied on.
///
/// Example:
///
/// ```no_run
Expand Down Expand Up @@ -461,13 +469,22 @@ impl fmt::Debug for Instant {
impl SystemTime {
/// An anchor in time which can be used to create new `SystemTime` instances or
/// learn about where in time a `SystemTime` lies.
//
// NOTE! this documentation is duplicated, here and in std::time::UNIX_EPOCH.
// The two copies are not quite identical, because of the difference in naming.
///
/// This constant is defined to be "1970-01-01 00:00:00 UTC" on all systems with
/// respect to the system clock. Using `duration_since` on an existing
/// `SystemTime` instance can tell how far away from this point in time a
/// measurement lies, and using `UNIX_EPOCH + duration` can be used to create a
/// `SystemTime` instance to represent another fixed point in time.
///
/// `duration_since(UNIX_EPOCH).unwrap().as_secs()`
/// returns a POSIX `time_t` (as a `u64`):
/// the number of non-leap seconds since the start of 1970 UTC.
/// This is the same time representation as used in many Internet protocols,
/// for example: JWT, CBOR, TOTP, certificate transparency and DNS TSIG/DNSSEC.
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I think enumerating these probably is over the line for too much documentation, since it might lead people to file PRs adding other protocols. I also would prefer saying "This is $description, same as $POSIX_equivalent" rather than the other way around.

So my suggesion would be for this (and the other) comment to be more like:

duration_since(UNIX_EPOCH).unwrap().as_secs() returns the number of non-leap seconds since the start of 1970 UTC, as a u64. This is the same time representation as used in POSIX time_t, as well as a wide variety of internet protocols.

(Note: I'm not tied to precise wording except for the bits I mentioned above). Thoughts?

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I'm OK with these suggestions and have adopted them. Thanks.

///
/// # Examples
///
/// ```no_run
Expand Down Expand Up @@ -617,13 +634,22 @@ impl fmt::Debug for SystemTime {

/// An anchor in time which can be used to create new `SystemTime` instances or
/// learn about where in time a `SystemTime` lies.
//
// NOTE! this documentation is duplicated, here and in SystemTime::UNIX_EPOCH.
// The two copies are not quite identical, because of the difference in naming.
///
/// This constant is defined to be "1970-01-01 00:00:00 UTC" on all systems with
/// respect to the system clock. Using `duration_since` on an existing
/// [`SystemTime`] instance can tell how far away from this point in time a
/// measurement lies, and using `UNIX_EPOCH + duration` can be used to create a
/// [`SystemTime`] instance to represent another fixed point in time.
///
/// `duration_since(UNIX_EPOCH).unwrap().as_secs()`
/// returns a POSIX `time_t` (as a `u64`):
/// the number of non-leap seconds since the start of 1970 UTC.
/// This is the same time representation as used in many Internet protocols,
/// for example: JWT, CBOR, TOTP, certificate transparency and DNS TSIG/DNSSEC.
///
/// # Examples
///
/// ```no_run
Expand Down