Fixed #61.

Technically it was a problem of `DateTime::date` with a lax test.
Documented the intention and guarantees on `Date` as a result.

src/date.rs

@@ -21,6 +21,28 @@ use datetime::DateTime;
 use format::{Item, DelayedFormat, StrftimeItems};
 
 /// ISO 8601 calendar date with time zone.
+///
+/// This type should be considered ambiguous at best,
+/// due to the inherent lack of precision required for the time zone resolution.
+/// There are some guarantees on the usage of `Date<Tz>`:
+///
+/// - If properly constructed via `TimeZone::ymd` and others without an error,
+///   the corresponding local date should exist for at least a moment.
+///   (It may still have a gap from the offset changes.)
+///
+/// - The `TimeZone` is free to assign *any* `Offset` to the local date,
+///   as long as that offset did occur in given day.
+///   For example, if `2015-03-08T01:59-08:00` is followed by `2015-03-08T03:00-07:00`,
+///   it may produce either `2015-03-08-08:00` or `2015-03-08-07:00`
+///   but *not* `2015-03-08+00:00` and others.
+///
+/// - Once constructed as a full `DateTime`,
+///   `DateTime::date` and other associated methods should return those for the original `Date`.
+///   For example, if `dt = tz.ymd(y,m,d).hms(h,n,s)` were valid, `dt.date() == tz.ymd(y,m,d)`.
+///
+/// - The date is timezone-agnostic up to one day (i.e. practically always),
+///   so the local date and UTC date should be equal for most cases
+///   even though the raw calculation between `NaiveDate` and `Duration` may not.
 #[derive(Clone)]
 #[cfg_attr(feature = "rustc-serialize", derive(RustcEncodable, RustcDecodable))]
 pub struct Date<Tz: TimeZone> {

src/datetime.rs

@@ -43,7 +43,7 @@ impl<Tz: TimeZone> DateTime<Tz> {
     /// Retrieves a date component.
     #[inline]
     pub fn date(&self) -> Date<Tz> {
-        Date::from_utc(self.datetime.date().clone(), self.offset.clone())
+        Date::from_utc(self.naive_local().date(), self.offset.clone())
     }
 
     /// Retrieves a time component.
@@ -429,6 +429,7 @@ mod tests {
     use super::DateTime;
     use Datelike;
     use naive::time::NaiveTime;
+    use naive::date::NaiveDate;
     use duration::Duration;
     use offset::TimeZone;
     use offset::utc::UTC;
@@ -481,9 +482,27 @@ mod tests {
     }
 
     #[test]
-    fn test_datetime_time() {
+    fn test_datetime_date_and_time() {
-        assert_eq!(FixedOffset::east(5*60*60).ymd(2014, 5, 6).and_hms(7, 8, 9).time(),
+        let tz = FixedOffset::east(5*60*60);
-                   NaiveTime::from_hms(7, 8, 9));
+        let d = tz.ymd(2014, 5, 6).and_hms(7, 8, 9);
+        assert_eq!(d.time(), NaiveTime::from_hms(7, 8, 9));
+        assert_eq!(d.date(), tz.ymd(2014, 5, 6));
+        assert_eq!(d.date().naive_local(), NaiveDate::from_ymd(2014, 5, 6));
+        assert_eq!(d.date().and_time(d.time()), Some(d));
+
+        let tz = FixedOffset::east(4*60*60);
+        let d = tz.ymd(2016, 5, 4).and_hms(3, 2, 1);
+        assert_eq!(d.time(), NaiveTime::from_hms(3, 2, 1));
+        assert_eq!(d.date(), tz.ymd(2016, 5, 4));
+        assert_eq!(d.date().naive_local(), NaiveDate::from_ymd(2016, 5, 4));
+        assert_eq!(d.date().and_time(d.time()), Some(d));
+
+        let tz = FixedOffset::west(13*60*60);
+        let d = tz.ymd(2017, 8, 9).and_hms(12, 34, 56);
+        assert_eq!(d.time(), NaiveTime::from_hms(12, 34, 56));
+        assert_eq!(d.date(), tz.ymd(2017, 8, 9));
+        assert_eq!(d.date().naive_local(), NaiveDate::from_ymd(2017, 8, 9));
+        assert_eq!(d.date().and_time(d.time()), Some(d));
     }
 
     #[test]