diff --git a/PR.md b/PR.md
new file mode 100644
index 0000000..b8dcd12
--- /dev/null
+++ b/PR.md
@@ -0,0 +1,97 @@
+# fix: emit full seconds in Instant and ZonedDateTime toISOString (RFC 3339)
+
+Fixes #110
+
+## Problem
+
+`Instant::toISOString()` (and therefore `Instant::jsonSerialize()` / `(string)`) and
+`ZonedDateTime::toISOString()` emit a compact time format that omits the seconds when they
+are zero, e.g.:
+
+```
+2023-11-14T22:14Z            // whole minute
+1970-01-01T00:00Z            // epoch
+2023-11-14T23:00Z            // whole hour
+```
+
+This is a valid ISO 8601 time, but it is **not** a valid RFC 3339 date-time. RFC 3339
+section 5.6 defines `full-time` as `partial-time time-offset` where
+`partial-time = time-hour ":" time-minute ":" time-second [time-secfrac]` — the seconds
+component is mandatory, even when it is `00`. `Instant` and `ZonedDateTime` are the types
+meant for interoperability (JSON serialization, wire formats), so they should always emit a
+value other systems can parse as RFC 3339. Producing `2023-11-14T22:14Z` breaks strict
+RFC 3339 parsers.
+
+## Root cause
+
+The compact behavior is intentional and documented in
+`src/LocalTime.php:639` (`toISOString()` — "the shortest that outputs the full value of the
+time"). That is correct for `LocalTime` and is left untouched.
+
+The bug is that the interop types inherit that compact form:
+
+- `src/ZonedDateTime.php:740` `toISOString()` built the string from
+  `$this->localDateTime`, whose `__toString()` delegates to `LocalTime::toISOString()`
+  (the compact formatter).
+- `src/Instant.php:362` `toISOString()` returns `(string) ZonedDateTime::ofInstant(...)`,
+  so it inherited the same compact time. `jsonSerialize()` and `__toString()` both route
+  through `toISOString()`.
+
+## The fix
+
+`ZonedDateTime::toISOString()` now builds the time portion explicitly with a mandatory
+`HH:MM:SS` component (keeping the existing optional fractional-seconds handling), instead of
+delegating to `LocalTime`'s compact formatter. `Instant::toISOString()` is fixed
+transitively because it delegates to `ZonedDateTime`; its PHPDoc now documents the RFC 3339
+guarantee. `LocalTime` is deliberately not changed.
+
+Output after the fix:
+
+```
+2023-11-14T22:14:00Z
+1970-01-01T00:00:00Z
+2023-11-14T23:00:00Z
+2023-11-14T23:14:00+01:00[Europe/Paris]
+```
+
+Fractional seconds and non-zero seconds are unchanged (e.g.
+`1970-01-01T00:00:00.000123456Z`, `1969-12-31T23:59:59Z`).
+
+## Why the test fixture changes are justified (BC break)
+
+Some existing fixtures pinned the buggy compact output and had to be updated to the
+RFC-3339-correct strings — this is the legitimate case of changing a maintainer's tests
+because the old expected values encode the bug being fixed:
+
+- `tests/InstantTest.php` `providerToString`: `[0, 0, '1970-01-01T00:00Z']` ->
+  `'1970-01-01T00:00:00Z'`.
+- `tests/ZonedDateTimeTest.php`: the DST plus/minus and interval result strings that landed
+  on whole minutes/hours (e.g. `2025-03-30T01:00+01:00[Europe/Prague]` ->
+  `2025-03-30T01:00:00+01:00[Europe/Prague]`).
+- `tests/IntervalTest.php`: one exception-message fixture (`1970-01-04T11:20Z` ->
+  `1970-01-04T11:20:00Z`).
+
+New assertions were added for whole-minute and whole-hour values to lock in the `:00`:
+
+- `tests/InstantTest.php`: `[1700000040, 0, '2023-11-14T22:14:00Z']` and
+  `[1700002800, 0, '2023-11-14T23:00:00Z']`.
+- `tests/ZonedDateTimeTest.php`: `['2000-01-20T12:34', '-07:00', '2000-01-20T12:34:00-07:00']`
+  and `['2000-01-20T12:00', 'America/Los_Angeles', '2000-01-20T12:00:00-08:00[America/Los_Angeles]']`.
+
+Because the serialized string representation changes, this is technically a backward
+compatibility break, matching the maintainer's note on issue #110: "this will target
+version 0.8.0 as this is technically a BC break." Parsing is unaffected — the parser still
+accepts the compact input form, so round-tripping and existing stored values keep working.
+
+## Test evidence
+
+Gate command:
+
+```
+composer install && vendor/bin/phpunit
+```
+
+Result: exit code `0` — `OK (7865 tests, 38307 assertions)`, 0 failures.
+
+Before the fix, `Instant::of(1700000040, 0)->toISOString()` returned `2023-11-14T22:14Z`;
+after the fix it returns `2023-11-14T22:14:00Z`.
diff --git a/src/Instant.php b/src/Instant.php
index 76c5afc..7cb42b4 100644
--- a/src/Instant.php
+++ b/src/Instant.php
@@ -357,6 +357,10 @@ final readonly class Instant implements JsonSerializable, Stringable
     /**
      * Returns the ISO 8601 representation of this instant.
      *
+     * The time part always includes the seconds, so that the output is a valid
+     * RFC 3339 date-time, as required by section 5.6 of that specification even
+     * when the seconds are zero.
+     *
      * @return non-empty-string
      */
     public function toISOString(): string
diff --git a/src/ZonedDateTime.php b/src/ZonedDateTime.php
index 099375a..576108c 100644
--- a/src/ZonedDateTime.php
+++ b/src/ZonedDateTime.php
@@ -735,11 +735,31 @@ final readonly class ZonedDateTime implements JsonSerializable, Stringable
     /**
      * Returns the ISO 8601 representation of this zoned date time.
      *
+     * Unlike {@see LocalTime::toISOString()}, the time part always includes the
+     * seconds, so that the output is a valid RFC 3339 date-time. This type is
+     * primarily meant for interoperability, and RFC 3339 section 5.6 requires
+     * the full `HH:MM:SS` time even when the seconds are zero.
+     *
      * @return non-empty-string
      */
     public function toISOString(): string
     {
-        $string = $this->localDateTime . $this->timeZoneOffset;
+        $time = $this->localDateTime->getTime();
+
+        $hour = $time->getHour();
+        $minute = $time->getMinute();
+        $second = $time->getSecond();
+        $nano = $time->getNano();
+
+        $string = $this->localDateTime->getDate()
+            . 'T'
+            . ($hour < 10 ? '0' . $hour : $hour)
+            . ':'
+            . ($minute < 10 ? '0' . $minute : $minute)
+            . ':'
+            . ($second < 10 ? '0' . $second : $second)
+            . ($nano !== 0 ? '.' . rtrim(str_pad((string) $nano, 9, '0', STR_PAD_LEFT), '0') : '')
+            . $this->timeZoneOffset;
 
         if ($this->timeZone instanceof TimeZoneRegion) {
             $string .= '[' . $this->timeZone . ']';
diff --git a/tests/InstantTest.php b/tests/InstantTest.php
index 1e3a042..efbc2c9 100644
--- a/tests/InstantTest.php
+++ b/tests/InstantTest.php
@@ -573,11 +573,11 @@ class InstantTest extends AbstractTestCase
     public static function providerGetIntervalTo(): array
     {
         return [
-            [1672567200, 0,       1672567200, 0,       '2023-01-01T10:00Z/2023-01-01T10:00Z'],
-            [1672567200, 0,       1672567210, 0,       '2023-01-01T10:00Z/2023-01-01T10:00:10Z'],
+            [1672567200, 0,       1672567200, 0,       '2023-01-01T10:00:00Z/2023-01-01T10:00:00Z'],
+            [1672567200, 0,       1672567210, 0,       '2023-01-01T10:00:00Z/2023-01-01T10:00:10Z'],
             [1672567200, 1000000, 1672567210, 2000000, '2023-01-01T10:00:00.001Z/2023-01-01T10:00:10.002Z'],
             [1672567200, 1,       1672567200, 9,       '2023-01-01T10:00:00.000000001Z/2023-01-01T10:00:00.000000009Z'],
-            [1672567200, 0,       1672653600, 0,       '2023-01-01T10:00Z/2023-01-02T10:00Z'],
+            [1672567200, 0,       1672653600, 0,       '2023-01-01T10:00:00Z/2023-01-02T10:00:00Z'],
         ];
     }
 
@@ -648,11 +648,14 @@ class InstantTest extends AbstractTestCase
             [-2000000000, 0, '1906-08-16T20:26:40Z'],
             [-1, 0, '1969-12-31T23:59:59Z'],
             [-1, 123, '1969-12-31T23:59:59.000000123Z'],
-            [0, 0, '1970-01-01T00:00Z'],
+            [0, 0, '1970-01-01T00:00:00Z'],
             [0, 123456, '1970-01-01T00:00:00.000123456Z'],
             [1, 0, '1970-01-01T00:00:01Z'],
             [1, 123456789, '1970-01-01T00:00:01.123456789Z'],
             [2000000000, 0, '2033-05-18T03:33:20Z'],
+            // whole minute and whole hour must keep the full ":00" seconds (RFC 3339 section 5.6)
+            [1700000040, 0, '2023-11-14T22:14:00Z'],
+            [1700002800, 0, '2023-11-14T23:00:00Z'],
         ];
     }
 
diff --git a/tests/IntervalTest.php b/tests/IntervalTest.php
index 9fc4724..5bbd8de 100644
--- a/tests/IntervalTest.php
+++ b/tests/IntervalTest.php
@@ -222,7 +222,7 @@ class IntervalTest extends AbstractTestCase
         $interval2 = Interval::of(Instant::of(300000), Instant::of(400000));
 
         $this->expectException(DateTimeException::class);
-        $this->expectExceptionMessage('Intervals "1970-01-02T03:46:40Z/1970-01-03T07:33:20Z" and "1970-01-04T11:20Z/1970-01-05T15:06:40Z" do not intersect.');
+        $this->expectExceptionMessage('Intervals "1970-01-02T03:46:40Z/1970-01-03T07:33:20Z" and "1970-01-04T11:20:00Z/1970-01-05T15:06:40Z" do not intersect.');
 
         $interval1->getIntersectionWith($interval2);
     }
diff --git a/tests/ZonedDateTimeTest.php b/tests/ZonedDateTimeTest.php
index b612225..9d85adb 100644
--- a/tests/ZonedDateTimeTest.php
+++ b/tests/ZonedDateTimeTest.php
@@ -436,7 +436,7 @@ class ZonedDateTimeTest extends AbstractTestCase
     {
         return [
             ['2018-07-21 14:09:10.23456', 'America/Los_Angeles', '2018-07-21T14:09:10.23456-07:00[America/Los_Angeles]'],
-            ['2019-01-21 17:59', 'America/Los_Angeles', '2019-01-21T17:59-08:00[America/Los_Angeles]'],
+            ['2019-01-21 17:59', 'America/Los_Angeles', '2019-01-21T17:59:00-08:00[America/Los_Angeles]'],
             ['2019-01-23 09:10:11.123', '+05:30', '2019-01-23T09:10:11.123+05:30'],
         ];
     }
@@ -753,12 +753,12 @@ class ZonedDateTimeTest extends AbstractTestCase
             ['2000-01-01T12:34:56.123456789-08:00[America/Los_Angeles]', 25, '2000-06-24T12:34:56.123456789-07:00[America/Los_Angeles]'],
 
             // https://github.com/brick/date-time/issues/115
-            ['2025-03-23T01:00+01:00[Europe/Prague]', 1, '2025-03-30T01:00+01:00[Europe/Prague]'],
-            ['2025-03-23T01:30+01:00[Europe/Prague]', 1, '2025-03-30T01:30+01:00[Europe/Prague]'],
-            ['2025-03-23T02:00+01:00[Europe/Prague]', 1, '2025-03-30T03:00+02:00[Europe/Prague]'],
-            ['2025-03-23T02:30+01:00[Europe/Prague]', 1, '2025-03-30T03:30+02:00[Europe/Prague]'],
-            ['2025-03-23T03:00+01:00[Europe/Prague]', 1, '2025-03-30T03:00+02:00[Europe/Prague]'],
-            ['2025-03-23T03:30+01:00[Europe/Prague]', 1, '2025-03-30T03:30+02:00[Europe/Prague]'],
+            ['2025-03-23T01:00+01:00[Europe/Prague]', 1, '2025-03-30T01:00:00+01:00[Europe/Prague]'],
+            ['2025-03-23T01:30+01:00[Europe/Prague]', 1, '2025-03-30T01:30:00+01:00[Europe/Prague]'],
+            ['2025-03-23T02:00+01:00[Europe/Prague]', 1, '2025-03-30T03:00:00+02:00[Europe/Prague]'],
+            ['2025-03-23T02:30+01:00[Europe/Prague]', 1, '2025-03-30T03:30:00+02:00[Europe/Prague]'],
+            ['2025-03-23T03:00+01:00[Europe/Prague]', 1, '2025-03-30T03:00:00+02:00[Europe/Prague]'],
+            ['2025-03-23T03:30+01:00[Europe/Prague]', 1, '2025-03-30T03:30:00+02:00[Europe/Prague]'],
         ];
     }
 
@@ -782,12 +782,12 @@ class ZonedDateTimeTest extends AbstractTestCase
             ['2000-04-03T12:34:56.123456789-07:00[America/Los_Angeles]', -2, '2000-04-01T12:34:56.123456789-08:00[America/Los_Angeles]'],
 
             // https://github.com/brick/date-time/issues/115
-            ['2025-03-29T01:00+01:00[Europe/Prague]', 1, '2025-03-30T01:00+01:00[Europe/Prague]'],
-            ['2025-03-29T01:30+01:00[Europe/Prague]', 1, '2025-03-30T01:30+01:00[Europe/Prague]'],
-            ['2025-03-29T02:00+01:00[Europe/Prague]', 1, '2025-03-30T03:00+02:00[Europe/Prague]'],
-            ['2025-03-29T02:30+01:00[Europe/Prague]', 1, '2025-03-30T03:30+02:00[Europe/Prague]'],
-            ['2025-03-29T03:00+01:00[Europe/Prague]', 1, '2025-03-30T03:00+02:00[Europe/Prague]'],
-            ['2025-03-29T03:30+01:00[Europe/Prague]', 1, '2025-03-30T03:30+02:00[Europe/Prague]'],
+            ['2025-03-29T01:00+01:00[Europe/Prague]', 1, '2025-03-30T01:00:00+01:00[Europe/Prague]'],
+            ['2025-03-29T01:30+01:00[Europe/Prague]', 1, '2025-03-30T01:30:00+01:00[Europe/Prague]'],
+            ['2025-03-29T02:00+01:00[Europe/Prague]', 1, '2025-03-30T03:00:00+02:00[Europe/Prague]'],
+            ['2025-03-29T02:30+01:00[Europe/Prague]', 1, '2025-03-30T03:30:00+02:00[Europe/Prague]'],
+            ['2025-03-29T03:00+01:00[Europe/Prague]', 1, '2025-03-30T03:00:00+02:00[Europe/Prague]'],
+            ['2025-03-29T03:30+01:00[Europe/Prague]', 1, '2025-03-30T03:30:00+02:00[Europe/Prague]'],
         ];
     }
 
@@ -815,12 +815,12 @@ class ZonedDateTimeTest extends AbstractTestCase
             ['2000-04-02T04:12:34.123456789-07:00[America/Los_Angeles]', 1, '2000-04-02T05:12:34.123456789-07:00[America/Los_Angeles]'],
 
             // https://github.com/brick/date-time/issues/115
-            ['2025-03-29T01:00+01:00[Europe/Prague]', 24, '2025-03-30T01:00+01:00[Europe/Prague]'],
-            ['2025-03-29T01:30+01:00[Europe/Prague]', 24, '2025-03-30T01:30+01:00[Europe/Prague]'],
-            ['2025-03-29T02:00+01:00[Europe/Prague]', 24, '2025-03-30T03:00+02:00[Europe/Prague]'],
-            ['2025-03-29T02:30+01:00[Europe/Prague]', 24, '2025-03-30T03:30+02:00[Europe/Prague]'],
-            ['2025-03-29T03:00+01:00[Europe/Prague]', 24, '2025-03-30T04:00+02:00[Europe/Prague]'],
-            ['2025-03-29T03:30+01:00[Europe/Prague]', 24, '2025-03-30T04:30+02:00[Europe/Prague]'],
+            ['2025-03-29T01:00+01:00[Europe/Prague]', 24, '2025-03-30T01:00:00+01:00[Europe/Prague]'],
+            ['2025-03-29T01:30+01:00[Europe/Prague]', 24, '2025-03-30T01:30:00+01:00[Europe/Prague]'],
+            ['2025-03-29T02:00+01:00[Europe/Prague]', 24, '2025-03-30T03:00:00+02:00[Europe/Prague]'],
+            ['2025-03-29T02:30+01:00[Europe/Prague]', 24, '2025-03-30T03:30:00+02:00[Europe/Prague]'],
+            ['2025-03-29T03:00+01:00[Europe/Prague]', 24, '2025-03-30T04:00:00+02:00[Europe/Prague]'],
+            ['2025-03-29T03:30+01:00[Europe/Prague]', 24, '2025-03-30T04:30:00+02:00[Europe/Prague]'],
         ];
     }
 
@@ -848,10 +848,10 @@ class ZonedDateTimeTest extends AbstractTestCase
             ['2000-04-02T03:01:01.654321-07:00[America/Los_Angeles]', 1, '2000-04-02T03:02:01.654321-07:00[America/Los_Angeles]'],
 
             // https://github.com/brick/date-time/issues/115
-            ['2025-03-30T01:30+01:00[Europe/Prague]', 50, '2025-03-30T03:20+02:00[Europe/Prague]'],
-            ['2025-03-30T01:30+01:00[Europe/Prague]', 100, '2025-03-30T04:10+02:00[Europe/Prague]'],
-            ['2025-03-30T03:20+02:00[Europe/Prague]', -50, '2025-03-30T01:30+01:00[Europe/Prague]'],
-            ['2025-03-30T04:10+02:00[Europe/Prague]', -100, '2025-03-30T01:30+01:00[Europe/Prague]'],
+            ['2025-03-30T01:30:00+01:00[Europe/Prague]', 50, '2025-03-30T03:20:00+02:00[Europe/Prague]'],
+            ['2025-03-30T01:30:00+01:00[Europe/Prague]', 100, '2025-03-30T04:10:00+02:00[Europe/Prague]'],
+            ['2025-03-30T03:20:00+02:00[Europe/Prague]', -50, '2025-03-30T01:30:00+01:00[Europe/Prague]'],
+            ['2025-03-30T04:10:00+02:00[Europe/Prague]', -100, '2025-03-30T01:30:00+01:00[Europe/Prague]'],
         ];
     }
 
@@ -878,10 +878,10 @@ class ZonedDateTimeTest extends AbstractTestCase
             ['2000-04-02T03:00:01.654321-07:00[America/Los_Angeles]', 1, '2000-04-02T03:00:02.654321-07:00[America/Los_Angeles]'],
 
             // https://github.com/brick/date-time/issues/115
-            ['2025-03-30T01:30+01:00[Europe/Prague]', 3000, '2025-03-30T03:20+02:00[Europe/Prague]'],
-            ['2025-03-30T01:30+01:00[Europe/Prague]', 6000, '2025-03-30T04:10+02:00[Europe/Prague]'],
-            ['2025-03-30T03:20+02:00[Europe/Prague]', -3000, '2025-03-30T01:30+01:00[Europe/Prague]'],
-            ['2025-03-30T04:10+02:00[Europe/Prague]', -6000, '2025-03-30T01:30+01:00[Europe/Prague]'],
+            ['2025-03-30T01:30:00+01:00[Europe/Prague]', 3000, '2025-03-30T03:20:00+02:00[Europe/Prague]'],
+            ['2025-03-30T01:30:00+01:00[Europe/Prague]', 6000, '2025-03-30T04:10:00+02:00[Europe/Prague]'],
+            ['2025-03-30T03:20:00+02:00[Europe/Prague]', -3000, '2025-03-30T01:30:00+01:00[Europe/Prague]'],
+            ['2025-03-30T04:10:00+02:00[Europe/Prague]', -6000, '2025-03-30T01:30:00+01:00[Europe/Prague]'],
         ];
     }
 
@@ -1086,6 +1086,9 @@ class ZonedDateTimeTest extends AbstractTestCase
         return [
             ['2000-01-20T12:34:56.123456789', 'America/Los_Angeles', '2000-01-20T12:34:56.123456789-08:00[America/Los_Angeles]'],
             ['2000-01-20T12:34:56.123456789', '-07:00', '2000-01-20T12:34:56.123456789-07:00'],
+            // whole minute and whole hour must keep the full ":00" seconds (RFC 3339 section 5.6)
+            ['2000-01-20T12:34', '-07:00', '2000-01-20T12:34:00-07:00'],
+            ['2000-01-20T12:00', 'America/Los_Angeles', '2000-01-20T12:00:00-08:00[America/Los_Angeles]'],
         ];
     }
 
@@ -1121,13 +1124,13 @@ class ZonedDateTimeTest extends AbstractTestCase
     public static function providerGetIntervalTo(): array
     {
         return [
-            ['2023-01-01T10:00:00Z',           '2023-01-01T10:00:00Z',           '2023-01-01T10:00Z/2023-01-01T10:00Z'],
-            ['2023-01-01T10:00:00Z',           '2023-01-01T10:00:10Z',           '2023-01-01T10:00Z/2023-01-01T10:00:10Z'],
+            ['2023-01-01T10:00:00Z',           '2023-01-01T10:00:00Z',           '2023-01-01T10:00:00Z/2023-01-01T10:00:00Z'],
+            ['2023-01-01T10:00:00Z',           '2023-01-01T10:00:10Z',           '2023-01-01T10:00:00Z/2023-01-01T10:00:10Z'],
             ['2023-01-01T10:00:00.001Z',       '2023-01-01T10:00:10.002Z',       '2023-01-01T10:00:00.001Z/2023-01-01T10:00:10.002Z'],
             ['2023-01-01T10:00:00.001Z',       '2023-01-01T13:00:10.002+03:00',  '2023-01-01T10:00:00.001Z/2023-01-01T10:00:10.002Z'],
             ['2023-01-01T10:00:00.001+03:00',  '2023-01-01T13:00:10.002+03:00',  '2023-01-01T07:00:00.001Z/2023-01-01T10:00:10.002Z'],
             ['2023-01-01T10:00:00.000000001Z', '2023-01-01T10:00:00.000000009Z', '2023-01-01T10:00:00.000000001Z/2023-01-01T10:00:00.000000009Z'],
-            ['2023-01-01T10:00:00Z',           '2023-01-02T10:00:00Z',           '2023-01-01T10:00Z/2023-01-02T10:00Z'],
+            ['2023-01-01T10:00:00Z',           '2023-01-02T10:00:00Z',           '2023-01-01T10:00:00Z/2023-01-02T10:00:00Z'],
         ];
     }