Add DateTime::<Utc>::from_timestamp_opt

This commit adds the new constructor `from_timestamp_opt` 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 using the `_opt` suffix for fallible functions. As panicking variants on invalid input are deprecated, no panicking variant is provided. See [this issue](chronotope#815) for discussion about error handling and panics.

Closes chronotope#832

src/datetime/mod.rs

@@ -202,6 +202,18 @@ 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`](#method.from_timestamp) or [`TimeZone::timestamp`].
+    ///
+    /// ```
+    /// 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(dt.timestamp(), dt.subsec_nanos()), dt);
+    /// ```
     #[inline]
     #[must_use]
     pub fn timestamp(&self) -> i64 {
@@ -552,6 +564,38 @@ 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(dt.timestamp(), dt.timestamp_subsec_nanos()), 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())