diff --git a/src/chrono/date.rs b/src/chrono/date.rs
index 9935a33..6611518 100644
--- a/src/chrono/date.rs
+++ b/src/chrono/date.rs
@@ -363,7 +363,13 @@ impl Datelike for DateZ {
 
 impl fmt::Show for DateZ {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f.buf, "{:04}-{:02}-{:02}", self.year(), self.month(), self.day())
+        let year = self.year();
+        if 0 <= year && year <= 9999 {
+            write!(f.buf, "{:04}-{:02}-{:02}", year, self.month(), self.day())
+        } else {
+            // ISO 8601 requires the explicit sign for out-of-range years
+            write!(f.buf, "{:+05}-{:02}-{:02}", year, self.month(), self.day())
+        }
     }
 }
 
@@ -569,6 +575,14 @@ mod tests {
                        DateZ::from_ymd(year - 1, 12, 31).unwrap().ndays_from_ce() + 1);
         }
     }
+
+    #[test]
+    fn test_date_fmt() {
+        assert_eq!(DateZ::from_ymd(2012,  3, 4).unwrap().to_str(),  ~"2012-03-04");
+        assert_eq!(DateZ::from_ymd(0,     3, 4).unwrap().to_str(),  ~"0000-03-04");
+        assert_eq!(DateZ::from_ymd(-307,  3, 4).unwrap().to_str(), ~"-0307-03-04");
+        assert_eq!(DateZ::from_ymd(12345, 3, 4).unwrap().to_str(), ~"+12345-03-04");
+    }
 }
 
 /**
diff --git a/src/chrono/duration.rs b/src/chrono/duration.rs
new file mode 100644
index 0000000..a692ff5
--- /dev/null
+++ b/src/chrono/duration.rs
@@ -0,0 +1,249 @@
+/*!
+ * ISO 8601 duration.
+ */
+
+use std::{fmt, num};
+
+static NANOS_PER_SEC: int = 1_000_000_000;
+static SECS_PER_DAY: int = 86400;
+
+#[deriving(Eq, TotalEq, Ord, TotalOrd)]
+pub struct Duration {
+    priv days: int,
+    priv secs: u32,
+    priv nanos: u32,
+}
+
+impl Duration {
+    pub fn new(days: int, secs: int, nanos: int) -> Option<Duration> {
+        let mut secs_ = nanos / NANOS_PER_SEC;
+        let mut nanos = nanos % NANOS_PER_SEC;
+        if nanos < 0 { secs_ -= 1; nanos += NANOS_PER_SEC; }
+        let secs = match secs.checked_add(&secs_) { Some(v) => v, None => return None };
+        let mut days_ = secs / SECS_PER_DAY;
+        let mut secs = secs % SECS_PER_DAY;
+        if secs < 0 { days_ -= 1; secs += SECS_PER_DAY; }
+        let days = match days.checked_add(&days_) { Some(v) => v, None => return None };
+        Some(Duration { days: days, secs: secs as u32, nanos: nanos as u32 })
+    }
+
+    #[inline]
+    pub fn zero() -> Duration {
+        Duration { days: 0, secs: 0, nanos: 0 }
+    }
+
+    #[inline]
+    pub fn weeks(weeks: int) -> Duration {
+        Duration::days(weeks * 7)
+    }
+
+    #[inline]
+    pub fn days(days: int) -> Duration {
+        Duration { days: days, secs: 0, nanos: 0 }
+    }
+
+    #[inline]
+    pub fn hours(hours: int) -> Duration {
+        Duration::seconds(hours * 3600)
+    }
+
+    #[inline]
+    pub fn minutes(mins: int) -> Duration {
+        Duration::seconds(mins * 60)
+    }
+
+    #[inline]
+    pub fn seconds(secs: int) -> Duration {
+        let mut days = secs / SECS_PER_DAY;
+        let mut secs = secs % SECS_PER_DAY;
+        if secs < 0 { days -= 1; secs += SECS_PER_DAY; }
+        Duration { secs: secs as u32, ..Duration::days(days) }
+    }
+
+    #[inline]
+    pub fn milliseconds(millis: int) -> Duration {
+        Duration::nanoseconds(millis * 1_000_000)
+    }
+
+    #[inline]
+    pub fn microseconds(micros: int) -> Duration {
+        Duration::nanoseconds(micros * 1_000)
+    }
+
+    #[inline]
+    pub fn nanoseconds(nanos: int) -> Duration {
+        let mut secs = nanos / NANOS_PER_SEC;
+        let mut nanos = nanos % NANOS_PER_SEC;
+        if nanos < 0 { secs -= 1; nanos += NANOS_PER_SEC; }
+        Duration { nanos: nanos as u32, ..Duration::seconds(secs) }
+    }
+}
+
+impl num::Zero for Duration {
+    #[inline]
+    fn zero() -> Duration {
+        Duration { days: 0, secs: 0, nanos: 0 }
+    }
+
+    #[inline]
+    fn is_zero(&self) -> bool {
+        self.days == 0 && self.secs == 0 && self.nanos == 0
+    }
+}
+
+impl Neg<Duration> for Duration {
+    fn neg(&self) -> Duration {
+        let mut days = -self.days;
+        let mut secs = -(self.secs as int);
+        let mut nanos = -(self.nanos as int);
+        if nanos < 0 {
+            nanos += NANOS_PER_SEC;
+            secs -= 1;
+        }
+        if secs < 0 {
+            secs += SECS_PER_DAY;
+            days -= 1;
+        }
+        Duration { days: days, secs: secs as u32, nanos: nanos as u32 }
+    }
+}
+
+impl Add<Duration,Duration> for Duration {
+    fn add(&self, rhs: &Duration) -> Duration {
+        let mut days = self.days + rhs.days;
+        let mut secs = self.secs + rhs.secs;
+        let mut nanos = self.nanos + rhs.nanos;
+        if nanos >= NANOS_PER_SEC as u32 {
+            nanos -= NANOS_PER_SEC as u32;
+            secs += 1;
+        }
+        if secs >= SECS_PER_DAY as u32 {
+            secs -= SECS_PER_DAY as u32;
+            days += 1;
+        }
+        Duration { days: days, secs: secs, nanos: nanos }
+    }
+}
+
+impl num::CheckedAdd for Duration {
+    fn checked_add(&self, rhs: &Duration) -> Option<Duration> {
+        let mut days = match self.days.checked_add(&rhs.days) { Some(v) => v, None => return None };
+        let mut secs = self.secs + rhs.secs;
+        let mut nanos = self.nanos + rhs.nanos;
+        if nanos >= NANOS_PER_SEC as u32 {
+            nanos -= NANOS_PER_SEC as u32;
+            secs += 1;
+        }
+        if secs >= SECS_PER_DAY as u32 {
+            secs -= SECS_PER_DAY as u32;
+            days = match days.checked_add(&1) { Some(v) => v, None => return None };
+        }
+        Some(Duration { days: days, secs: secs, nanos: nanos })
+    }
+}
+
+impl Sub<Duration,Duration> for Duration {
+    fn sub(&self, rhs: &Duration) -> Duration {
+        let mut days = self.days - rhs.days;
+        let mut secs = self.secs as int - rhs.secs as int;
+        let mut nanos = self.nanos as int - rhs.nanos as int;
+        if nanos < 0 {
+            nanos += NANOS_PER_SEC;
+            secs -= 1;
+        }
+        if secs < 0 {
+            secs += SECS_PER_DAY;
+            days -= 1;
+        }
+        Duration { days: days, secs: secs as u32, nanos: nanos as u32 }
+    }
+}
+
+impl num::CheckedSub for Duration {
+    fn checked_sub(&self, rhs: &Duration) -> Option<Duration> {
+        let mut days = match self.days.checked_sub(&rhs.days) { Some(v) => v, None => return None };
+        let mut secs = self.secs as int - rhs.secs as int;
+        let mut nanos = self.nanos as int - rhs.nanos as int;
+        if nanos < 0 {
+            nanos += NANOS_PER_SEC;
+            secs -= 1;
+        }
+        if secs < 0 {
+            secs += SECS_PER_DAY;
+            days = match days.checked_sub(&1) { Some(v) => v, None => return None };
+        }
+        Some(Duration { days: days, secs: secs as u32, nanos: nanos as u32 })
+    }
+}
+
+impl fmt::Show for Duration {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let hasdate = self.days != 0;
+        let hastime = (self.secs != 0 || self.nanos != 0) || !hasdate;
+
+        try!('P'.fmt(f));
+        if hasdate {
+            // technically speaking the negative part is not the valid ISO 8601,
+            // but we need to print it anyway.
+            try!(write!(f.buf, "{}D", self.days));
+        }
+        if hastime {
+            if self.nanos == 0 {
+                try!(write!(f.buf, "T{}S", self.secs));
+            } else if self.nanos % 1_000_000 == 0 {
+                try!(write!(f.buf, "T{},{:03}S", self.secs, self.nanos / 1_000_000));
+            } else if self.nanos % 1_000 == 0 {
+                try!(write!(f.buf, "T{},{:06}S", self.secs, self.nanos / 1_000));
+            } else {
+                try!(write!(f.buf, "T{},{:09}S", self.secs, self.nanos));
+            }
+        }
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::int;
+
+    #[test]
+    fn test_duration() {
+        assert_eq!(Duration::zero(), Duration::zero());
+        assert!(Duration::zero() != Duration::seconds(1));
+        assert_eq!(Duration::seconds(1) + Duration::seconds(2), Duration::seconds(3));
+        assert_eq!(Duration::seconds(86399) + Duration::seconds(4),
+                   Duration::days(1) + Duration::seconds(3));
+        assert_eq!(Duration::days(10) - Duration::seconds(1000), Duration::seconds(863000));
+        assert_eq!(Duration::days(10) - Duration::seconds(1000000), Duration::seconds(-136000));
+        assert_eq!(Duration::days(2) + Duration::seconds(86391) + Duration::nanoseconds(9876543210),
+                   Duration::days(3) + Duration::nanoseconds(876543210));
+        assert_eq!(-Duration::days(3), Duration::days(-3));
+        assert_eq!(-(Duration::days(3) + Duration::seconds(70)),
+                   Duration::days(-4) + Duration::seconds(86400-70));
+    }
+
+    #[test]
+    fn test_duration_checked_ops() {
+        assert_eq!(Duration::days(int::MAX).checked_add(&Duration::seconds(86399)),
+                   Some(Duration::days(int::MAX - 1) + Duration::seconds(86400+86399)));
+        assert!(Duration::days(int::MAX).checked_add(&Duration::seconds(86400)).is_none());
+
+        assert_eq!(Duration::days(int::MIN).checked_sub(&Duration::seconds(0)),
+                   Some(Duration::days(int::MIN)));
+        assert!(Duration::days(int::MIN).checked_sub(&Duration::seconds(1)).is_none());
+    }
+
+    #[test]
+    fn test_duration_fmt() {
+        assert_eq!(Duration::zero().to_str(), ~"PT0S");
+        assert_eq!(Duration::days(42).to_str(), ~"P42D");
+        assert_eq!(Duration::days(-42).to_str(), ~"P-42D");
+        assert_eq!(Duration::seconds(42).to_str(), ~"PT42S");
+        assert_eq!(Duration::milliseconds(42).to_str(), ~"PT0,042S");
+        assert_eq!(Duration::microseconds(42).to_str(), ~"PT0,000042S");
+        assert_eq!(Duration::nanoseconds(42).to_str(), ~"PT0,000000042S");
+        assert_eq!((Duration::days(7) + Duration::milliseconds(6543)).to_str(), ~"P7DT6,543S");
+    }
+}
+
diff --git a/src/chrono/lib.rs b/src/chrono/lib.rs
index 37ed50c..c8a9a64 100644
--- a/src/chrono/lib.rs
+++ b/src/chrono/lib.rs
@@ -3,6 +3,7 @@
 
 #[feature(globs, macro_rules)];
 
+pub mod duration;
 pub mod date;
 pub mod time;
 pub mod datetime;