Add DateTime::<Utc>::from_timestamp

This commit adds the new constructor `from_timestamp` to build a `DateTime<Utc>` from a UNIX timestamp.

Figuring out how to convert a timestamp into a `DateTime<Utc>` was a common issue:
- chronotope#88
- chronotope#200
- chronotope#832

This commit should make `DateTime<Utc>` creation more discoverable and intuitive.

This commit respects the current convention of preferring fallible functions. It avoids however the `_opt` suffix as there is no panicking variant. See [this issue](chronotope#815) for discussion about error handling and panics.

Closes chronotope#832

src/datetime/mod.rs

@@ -209,6 +209,18 @@ impl<Tz: TimeZone> DateTime<Tz> {
 
     /// Returns the number of non-leap seconds since January 1, 1970 0:00:00 UTC
     /// (aka "UNIX timestamp").
+    ///
+    /// The reverse operation of creating a [`DateTime`] from a timestamp can be performed
+    /// using [`from_timestamp_opt`](#method.from_timestamp_opt) or [`TimeZone::timestamp_opt`].
+    ///
+    /// ```
+    /// use chrono::{DateTime, TimeZone, Utc};
+    ///
+    /// let dt: DateTime<Utc> = Utc.with_ymd_and_hms(2015, 5, 15, 0, 0, 0).unwrap();
+    /// assert_eq!(dt.timestamp(), 1431648000);
+    ///
+    /// assert_eq!(DateTime::from_timestamp_opt(dt.timestamp(), dt.timestamp_subsec_nanos()).unwrap(), dt);
+    /// ```
     #[inline]
     #[must_use]
     pub fn timestamp(&self) -> i64 {
@@ -559,6 +571,38 @@ impl<Tz: TimeZone> DateTime<Tz> {
     pub const MAX_UTC: DateTime<Utc> = DateTime { datetime: NaiveDateTime::MAX, offset: Utc };
 }
 
+impl DateTime<Utc> {
+    /// Makes a new [`DateTime<Utc>`] from the number of non-leap seconds
+    /// since January 1, 1970 0:00:00 UTC (aka "UNIX timestamp")
+    /// and the number of nanoseconds since the last whole non-leap second.
+    ///
+    /// This is guaranteed to round-trip with regard to [`timestamp`](#method.timestamp) and
+    /// [`timestamp_subsec_nanos`](#method.timestamp).
+    ///
+    /// Returns `None` on out-of-range number of seconds and/or
+    /// invalid nanosecond, otherwise returns `Some(DateTime {...})`.
+    ///
+    /// If you need to create a `DateTime` with some other [`TimeZone`], use [`TimeZone::timestamp_opt`]
+    /// or [`DateTime::with_timezone`].
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use chrono::{DateTime, Utc};
+    ///
+    /// let dt: DateTime<Utc> = DateTime::<Utc>::from_timestamp_opt(1431648000, 0).expect("invalid timestamp");
+    ///
+    /// assert_eq!(dt.to_string(), "2015-05-15 00:00:00 UTC");
+    /// assert_eq!(DateTime::from_timestamp_opt(dt.timestamp(), dt.timestamp_subsec_nanos()).unwrap(), dt);
+    /// ```
+    #[inline]
+    #[must_use]
+    pub fn from_timestamp_opt(secs: i64, nsecs: u32) -> Option<Self> {
+        NaiveDateTime::from_timestamp_opt(secs, nsecs)
+            .map(|ndt| DateTime::from_naive_utc_and_offset(ndt, Utc))
+    }
+}
+
 impl Default for DateTime<Utc> {
     fn default() -> Self {
         Utc.from_utc_datetime(&NaiveDateTime::default())

src/lib.rs

@@ -309,7 +309,7 @@
 //!
 //! ### Conversion from and to EPOCH timestamps
 //!
-//! Use [`Utc.timestamp(seconds, nanoseconds)`](./offset/trait.TimeZone.html#method.timestamp)
+//! Use [`DateTime::from_timestamp_opt(seconds, nanoseconds)`](./struct.DateTime.html#method.from_timestamp_opt)
 //! to construct a [`DateTime<Utc>`](./struct.DateTime.html) from a UNIX timestamp
 //! (seconds, nanoseconds that passed since January 1st 1970).
 //!
@@ -321,10 +321,10 @@
 #![cfg_attr(not(feature = "std"), doc = "```ignore")]
 #![cfg_attr(feature = "std", doc = "```rust")]
 //! // We need the trait in scope to use Utc::timestamp().
-//! use chrono::{DateTime, TimeZone, Utc};
+//! use chrono::{DateTime, FixedOffset, Utc};
 //!
 //! // Construct a datetime from epoch:
-//! let dt = Utc.timestamp_opt(1_500_000_000, 0).unwrap();
+//! let dt: DateTime<Utc> = DateTime::from_timestamp_opt(1_500_000_000, 0).unwrap();
 //! assert_eq!(dt.to_rfc2822(), "Fri, 14 Jul 2017 02:40:00 +0000");
 //!
 //! // Get epoch value from a datetime: