fmt: fix buffer sizes when printing durations

The ISO 8601 buffer for durations was sized for `SignedDuration::MIN`,
but the longest output is a negative duration with 16 digits of hours
and two digit minutes and seconds, e.g.,
`-PT2562047788015214H59M59.999999999S`, which is 36 bytes.

Similarly, the friendly format buffer for `std::time::Duration` didn't
account for the `+` prefix written by `Direction::ForceSign`.

Fixes #654

CHANGELOG.md

@@ -1,5 +1,15 @@
 # CHANGELOG
 
+0.2.39 (TBD)
+============
+TODO
+
+Bug fixes:
+
+* [#654](https://github.com/BurntSushi/jiff/issues/654):
+Fixes a panic when printing some very large durations.
+
+
 0.2.38 (2026-10-06)
 ===================
 This release updates Jiff's bundled copy of the [IANA Time Zone Database]

crates/jiff/src/fmt/friendly/printer.rs

@@ -50,8 +50,9 @@ const MAX_SIGNED_DURATION_LEN: usize = 194;
 ///
 /// Note that this is shorter than `MAX_SIGNED_DURATION` because one cannot
 /// get a negative formatted duration from a `std::time::Duration`. So there's
-/// no way to get an ` ago` suffix (or a `-` prefix).
-const MAX_UNSIGNED_DURATION_LEN: usize = 190;
+/// no way to get an ` ago` suffix (or a `-` prefix). It can still get a `+`
+/// prefix with `Direction::ForceSign` though, which is counted here.
+const MAX_UNSIGNED_DURATION_LEN: usize = 191;
 
 /// Configuration for [`SpanPrinter::designator`].
 ///
@@ -4169,5 +4170,11 @@ mod tests {
             p.unsigned_duration_to_string(&udur),
             @"00005124095576030430 hours, 00000000000000000059 minutes, 00000000000000000059 seconds, 00000000000000000999 milliseconds, 00000000000000000999 microseconds, 00000000000000000999 nanoseconds",
         );
+
+        let p = p.direction(Direction::ForceSign);
+        insta::assert_snapshot!(
+            p.unsigned_duration_to_string(&udur),
+            @"+00005124095576030430 hours, 00000000000000000059 minutes, 00000000000000000059 seconds, 00000000000000000999 milliseconds, 00000000000000000999 microseconds, 00000000000000000999 nanoseconds",
+        );
     }
 }

crates/jiff/src/fmt/temporal/printer.rs

@@ -134,9 +134,11 @@ const MAX_SPAN_LEN: usize = 78;
 /// Defines the maximum possible length (in bytes) of a duration printed in the
 /// Temporal ISO 8601 format.
 ///
-/// This applies to both signed and unsigned durations. Unsigned durations have
-/// one more digit, but signed durations can have a negative sign.
-const MAX_DURATION_LEN: usize = 35;
+/// This applies to both signed and unsigned durations. The longest one isn't
+/// at either extreme. It's a negative duration with 16 digits of hours and
+/// two digit minutes and seconds, e.g.,
+/// `-PT2562047788015214H59M59.999999999S`.
+const MAX_DURATION_LEN: usize = 36;
 
 #[derive(Clone, Debug)]
 pub(super) struct DateTimePrinter {
@@ -1897,6 +1899,15 @@ mod tests {
             p(i64::MAX, 999_999_999),
             @"PT2562047788015215H30M7.999999999S",
         );
+
+        insta::assert_snapshot!(
+            p(-(2_562_047_788_015_214 * 3_600 + 3_599), -999_999_999),
+            @"-PT2562047788015214H59M59.999999999S",
+        );
+        insta::assert_snapshot!(
+            p(2_562_047_788_015_214 * 3_600 + 3_599, 999_999_999),
+            @"PT2562047788015214H59M59.999999999S",
+        );
     }
 
     #[cfg(not(miri))]
@@ -1929,5 +1940,9 @@ mod tests {
             p(u64::MAX, 999_999_999),
             @"PT5124095576030431H15.999999999S",
         );
+        insta::assert_snapshot!(
+            p(5_124_095_576_030_430 * 3_600 + 3_599, 999_999_999),
+            @"PT5124095576030430H59M59.999999999S",
+        );
     }
 }