Improve docs around constructing DateTime objects

This provides examples for most of the constructor-like methods on
`TimeZone`, examples on the various `Offset` impls, and links
`NaiveDateTime` to `TimeZone` so that it's more obvious how you're
supposed to do things.

This is related to #88, which is something that I ran into when I
started using rust-chrono.

src/datetime.rs

@@ -23,6 +23,10 @@ use format::{Item, Numeric, Pad, Fixed};
 use format::{parse, Parsed, ParseError, ParseResult, DelayedFormat, StrftimeItems};
 
 /// ISO 8601 combined date and time with time zone.
+///
+/// There are some constructors implemented here (the `from_*` methods), but
+/// the general-purpose constructors are all via the methods on the
+/// [`TimeZone`](../trait.TimeZone.html) implementations.
 #[derive(Clone)]
 pub struct DateTime<Tz: TimeZone> {
     datetime: NaiveDateTime,
@@ -32,6 +36,15 @@ pub struct DateTime<Tz: TimeZone> {
 impl<Tz: TimeZone> DateTime<Tz> {
     /// Makes a new `DateTime` with given *UTC* datetime and offset.
     /// The local datetime should be constructed via the `TimeZone` trait.
+    ///
+    /// # Example
+    ///
+    /// ~~~~
+    /// use chrono::{DateTime, TimeZone, NaiveDateTime, UTC};
+    ///
+    /// let dt = DateTime::<UTC>::from_utc(NaiveDateTime::from_timestamp(61, 0), UTC);
+    /// assert_eq!(UTC.timestamp(61, 0), dt);
+    /// ~~~~
     //
     // note: this constructor is purposedly not named to `new` to discourage the direct usage.
     #[inline]

src/naive/datetime.rs

@@ -82,9 +82,12 @@ impl NaiveDateTime {
     /// since the midnight UTC on January 1, 1970 (aka "UNIX timestamp")
     /// and the number of nanoseconds since the last whole non-leap second.
     ///
-    /// The nanosecond part can exceed 1,000,000,000
+    /// For a non-naive version of this function see
-    /// in order to represent the [leap second](../time/index.html#leap-second-handling).
+    /// [`TimeZone::timestamp`](../../offset/trait.TimeZone.html#method.timestamp).
-    /// (The true "UNIX timestamp" cannot represent a leap second unambiguously.)
+    ///
+    /// The nanosecond part can exceed 1,000,000,000 in order to represent the
+    /// [leap second](../time/index.html#leap-second-handling). (The true "UNIX
+    /// timestamp" cannot represent a leap second unambiguously.)
     ///
     /// Panics on the out-of-range number of seconds and/or invalid nanosecond.
     ///

src/offset/fixed.rs

@@ -15,6 +15,11 @@ use naive::datetime::NaiveDateTime;
 use super::{TimeZone, Offset, LocalResult};
 
 /// The time zone with fixed offset, from UTC-23:59:59 to UTC+23:59:59.
+///
+/// Using the [`TimeZone`](../../../chrono/offset/trait.TimeZone.html) methods
+/// on a `FixedOffset` struct is the preferred way to construct
+/// `DateTime<FixedOffset>` instances. See the [`east`](#method.east) and
+/// [`west`](#method.west) methods for examples.
 #[derive(Copy, Clone, PartialEq, Eq)]
 pub struct FixedOffset {
     local_minus_utc: i32,
@@ -42,6 +47,16 @@ impl FixedOffset {
     /// The negative `secs` means the Western Hemisphere.
     ///
     /// Panics on the out-of-bound `secs`.
+    ///
+    /// # Example
+    ///
+    /// ~~~~
+    /// use chrono::{FixedOffset, TimeZone};
+    /// let hour = 3600;
+    /// let datetime = FixedOffset::east(5 * hour).ymd(2016, 11, 08)
+    ///                                           .and_hms(0, 0, 0);
+    /// assert_eq!(&datetime.to_rfc3339(), "2016-11-08T00:00:00+05:00")
+    /// ~~~~
     pub fn east(secs: i32) -> FixedOffset {
         FixedOffset::east_opt(secs).expect("FixedOffset::east out of bounds")
     }
@@ -62,6 +77,16 @@ impl FixedOffset {
     /// The negative `secs` means the Eastern Hemisphere.
     ///
     /// Panics on the out-of-bound `secs`.
+    ///
+    /// # Example
+    ///
+    /// ~~~~
+    /// use chrono::{FixedOffset, TimeZone};
+    /// let hour = 3600;
+    /// let datetime = FixedOffset::west(5 * hour).ymd(2016, 11, 08)
+    ///                                           .and_hms(0, 0, 0);
+    /// assert_eq!(&datetime.to_rfc3339(), "2016-11-08T00:00:00-05:00")
+    /// ~~~~
     pub fn west(secs: i32) -> FixedOffset {
         FixedOffset::west_opt(secs).expect("FixedOffset::west out of bounds")
     }

src/offset/local.rs

@@ -69,6 +69,19 @@ fn datetime_to_timespec(d: &NaiveDateTime, local: bool) -> stdtime::Timespec {
 }
 
 /// The local timescale. This is implemented via the standard `time` crate.
+///
+/// Using the [`TimeZone`](../../../chrono/offset/trait.TimeZone.html) methods
+/// on the Local struct is the preferred way to construct `DateTime<Local>`
+/// instances.
+///
+/// # Example
+///
+/// ~~~~
+/// use chrono::{Local, DateTime, TimeZone};
+///
+/// let dt: DateTime<Local> = Local::now();
+/// let dt: DateTime<Local> = Local.timestamp(0, 0);
+/// ~~~~
 #[derive(Copy, Clone)]
 #[cfg_attr(feature = "rustc-serialize", derive(RustcEncodable, RustcDecodable))]
 pub struct Local;

src/offset/mod.rs

@@ -5,11 +5,12 @@
 /*!
  * The time zone, which calculates offsets from the local time to UTC.
  *
- * There are three operations provided by the `TimeZone` trait:
+ * There are four operations provided by the `TimeZone` trait:
  *
  * 1. Converting the local `NaiveDateTime` to `DateTime<Tz>`
  * 2. Converting the UTC `NaiveDateTime` to `DateTime<Tz>`
  * 3. Converting `DateTime<Tz>` to the local `NaiveDateTime`
+ * 4. Constructing `DateTime<Tz>` objects from various offsets
  *
  * 1 is used for constructors. 2 is used for the `with_timezone` method of date and time types.
  * 3 is used for other methods, e.g. `year()` or `format()`, and provided by an associated type
@@ -178,6 +179,9 @@ pub trait Offset: Sized + Clone + fmt::Debug {
 }
 
 /// The time zone.
+///
+/// The methods here are the primarily constructors for [`Date`](../date/struct.Date.html) and
+/// [`DateTime`](../datetime/struct.DateTime.html) types.
 pub trait TimeZone: Sized + Clone {
     /// An associated offset type.
     /// This type is used to store the actual offset in date and time types.
@@ -191,6 +195,14 @@ pub trait TimeZone: Sized + Clone {
     /// but it will propagate to the `DateTime` values constructed via this date.
     ///
     /// Panics on the out-of-range date, invalid month and/or day.
+    ///
+    /// # Example
+    ///
+    /// ~~~~
+    /// use chrono::{UTC, TimeZone};
+    ///
+    /// assert_eq!(UTC.ymd(2015, 5, 15).to_string(), "2015-05-15UTC");
+    /// ~~~~
     fn ymd(&self, year: i32, month: u32, day: u32) -> Date<Self> {
         self.ymd_opt(year, month, day).unwrap()
     }
@@ -202,6 +214,15 @@ pub trait TimeZone: Sized + Clone {
     /// but it will propagate to the `DateTime` values constructed via this date.
     ///
     /// Returns `None` on the out-of-range date, invalid month and/or day.
+    ///
+    /// # Example
+    ///
+    /// ~~~~
+    /// use chrono::{UTC, LocalResult, TimeZone};
+    ///
+    /// assert_eq!(UTC.ymd_opt(2015, 5, 15).unwrap().to_string(), "2015-05-15UTC");
+    /// assert_eq!(UTC.ymd_opt(2000, 0, 0), LocalResult::None);
+    /// ~~~~
     fn ymd_opt(&self, year: i32, month: u32, day: u32) -> LocalResult<Date<Self>> {
         match NaiveDate::from_ymd_opt(year, month, day) {
             Some(d) => self.from_local_date(&d),
@@ -216,6 +237,14 @@ pub trait TimeZone: Sized + Clone {
     /// but it will propagate to the `DateTime` values constructed via this date.
     ///
     /// Panics on the out-of-range date and/or invalid DOY.
+    ///
+    /// # Example
+    ///
+    /// ~~~~
+    /// use chrono::{UTC, TimeZone};
+    ///
+    /// assert_eq!(UTC.yo(2015, 135).to_string(), "2015-05-15UTC");
+    /// ~~~~
     fn yo(&self, year: i32, ordinal: u32) -> Date<Self> {
         self.yo_opt(year, ordinal).unwrap()
     }
@@ -243,6 +272,14 @@ pub trait TimeZone: Sized + Clone {
     /// but it will propagate to the `DateTime` values constructed via this date.
     ///
     /// Panics on the out-of-range date and/or invalid week number.
+    ///
+    /// # Example
+    ///
+    /// ~~~~
+    /// use chrono::{UTC, Weekday, TimeZone};
+    ///
+    /// assert_eq!(UTC.isoywd(2015, 20, Weekday::Fri).to_string(), "2015-05-15UTC");
+    /// ~~~~
     fn isoywd(&self, year: i32, week: u32, weekday: Weekday) -> Date<Self> {
         self.isoywd_opt(year, week, weekday).unwrap()
     }
@@ -268,6 +305,14 @@ pub trait TimeZone: Sized + Clone {
     /// and the number of nanoseconds since the last whole non-leap second.
     ///
     /// Panics on the out-of-range number of seconds and/or invalid nanosecond.
+    ///
+    /// # Example
+    ///
+    /// ~~~~
+    /// use chrono::{UTC, TimeZone};
+    ///
+    /// assert_eq!(UTC.timestamp(1431648000, 0).to_string(), "2015-05-15 00:00:00 UTC");
+    /// ~~~~
     fn timestamp(&self, secs: i64, nsecs: u32) -> DateTime<Self> {
         self.timestamp_opt(secs, nsecs).unwrap()
     }

src/offset/utc.rs

@@ -18,6 +18,21 @@ use super::{TimeZone, Offset, LocalResult};
 
 /// The UTC time zone. This is the most efficient time zone when you don't need the local time.
 /// It is also used as an offset (which is also a dummy type).
+///
+/// Using the [`TimeZone`](../../../chrono/offset/trait.TimeZone.html) methods
+/// on the UTC struct is the preferred way to construct `DateTime<UTC>`
+/// instances.
+///
+/// # Example
+///
+/// ~~~~
+/// use chrono::{DateTime, TimeZone, NaiveDateTime, UTC};
+///
+/// let dt = DateTime::<UTC>::from_utc(NaiveDateTime::from_timestamp(61, 0), UTC);
+///
+/// assert_eq!(UTC.timestamp(61, 0), dt);
+/// assert_eq!(UTC.ymd(1970, 1, 1).and_hms(0, 1, 1), dt);
+/// ~~~~
 #[derive(Copy, Clone, PartialEq, Eq)]
 #[cfg_attr(feature = "rustc-serialize", derive(RustcEncodable, RustcDecodable))]
 pub struct UTC;