Add Display impls for ExtendedDateTime and ParsedDateTime

Provide a standard formatting path so callers (and tests) can use
`.to_string()` instead of hand-rolling offset formatting everywhere.

src/extended.rs

@@ -1,6 +1,8 @@
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
 
+use std::fmt;
+
 use crate::GNU_MAX_YEAR;
 
 const SECONDS_PER_DAY: i64 = 86_400;
@@ -289,6 +291,20 @@ impl ExtendedDateTime {
     }
 }
 
+impl fmt::Display for ExtendedDateTime {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let sign = if self.offset_seconds < 0 { '-' } else { '+' };
+        let abs = self.offset_seconds.unsigned_abs();
+        let oh = abs / 3600;
+        let om = (abs % 3600) / 60;
+        write!(
+            f,
+            "{:04}-{:02}-{:02} {:02}:{:02}:{:02}{}{:02}:{:02}",
+            self.year, self.month, self.day, self.hour, self.minute, self.second, sign, oh, om,
+        )
+    }
+}
+
 pub fn is_leap_year(year: u32) -> bool {
     (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)
 }

src/lib.rs

@@ -55,6 +55,15 @@ impl ParsedDateTime {
     }
 }
 
+impl fmt::Display for ParsedDateTime {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            ParsedDateTime::InRange(z) => write!(f, "{}", z.strftime("%Y-%m-%d %H:%M:%S%:z")),
+            ParsedDateTime::Extended(dt) => write!(f, "{dt}"),
+        }
+    }
+}
+
 impl PartialEq<Zoned> for ParsedDateTime {
     fn eq(&self, other: &Zoned) -> bool {
         match self {