fix: handle UTC offset timezones

lib/TimezoneGuesser/FindFromOffset.php

@@ -24,6 +24,24 @@ public function find(string $tzid, ?bool $failIfUncertain = false): ?\DateTimeZo
             // @codeCoverageIgnoreEnd
         }
 
+        if (0 === strncasecmp($tzid, 'UTC', 3)) {
+            $offset = substr($tzid, 3);
+            if (6 === strlen($offset) && ':' === $offset[3]) {
+                $offset = substr($offset, 0, 3).substr($offset, 4);
+            }
+
+            if (5 !== strlen($offset)
+                || ('+' !== $offset[0] && '-' !== $offset[0])
+                || 4 !== strspn(substr($offset, 1), '0123456789')
+                || (int) substr($offset, 1, 2) > 23
+                || (int) substr($offset, 3, 2) > 59
+            ) {
+                return null;
+            }
+
+            return new \DateTimeZone($offset);
+        }
+
         return null;
     }
 }

tests/VObject/TimeZoneUtilTest.php

@@ -221,6 +221,72 @@ public function testTimezoneOffset(): void
         self::assertEquals($ex->getName(), $tz->getName());
     }
 
+    #[DataProvider('getValidUtcOffsets')]
+    public function testValidUtcOffset(string $tzid, string $expectedOffset): void
+    {
+        $tz = TimeZoneUtil::getTimeZone($tzid, null, true);
+
+        self::assertSame($expectedOffset, $tz->getName());
+    }
+
+    public static function getValidUtcOffsets(): array
+    {
+        return [
+            ['UTC-04:00', '-04:00'],
+            ['UTC-0400', '-04:00'],
+            ['UTC+05:30', '+05:30'],
+            ['UTC+0545', '+05:45'],
+            ['utc-03:30', '-03:30'],
+            ['UTC+00:00', '+00:00'],
+            ['UTC-00:00', '+00:00'],
+            ['UTC+23:59', '+23:59'],
+        ];
+    }
+
+    #[DataProvider('getInvalidUtcOffsets')]
+    public function testInvalidUtcOffset(string $tzid): void
+    {
+        self::assertSame(date_default_timezone_get(), TimeZoneUtil::getTimeZone($tzid)->getName());
+
+        $this->expectException(\InvalidArgumentException::class);
+        TimeZoneUtil::getTimeZone($tzid, null, true);
+    }
+
+    public static function getInvalidUtcOffsets(): array
+    {
+        return [
+            ['UTC-24:00'],
+            ['UTC+04:60'],
+            ['UTC+4:00'],
+            ['UTC+04'],
+            ['UTC04:00'],
+            ['UTC+ab:cd'],
+            ['UTC+04:00extra'],
+            ["UTC+04:00\n"],
+            ['UTC+04::00'],
+            ['UTC+'],
+        ];
+    }
+
+    public function testParseUtcOffsetDateTime(): void
+    {
+        $calendar = Reader::read(<<<ICS
+BEGIN:VCALENDAR
+VERSION:2.0
+BEGIN:VEVENT
+UID:fixed-offset
+DTSTART;TZID="UTC-04:00":20260728T111500
+END:VEVENT
+END:VCALENDAR
+ICS
+        );
+
+        $start = $calendar->VEVENT->DTSTART->getDateTime();
+
+        self::assertSame('2026-07-28 11:15:00 -04:00', $start->format('Y-m-d H:i:s P'));
+        self::assertSame((new \DateTimeImmutable('2026-07-28T15:15:00Z'))->getTimestamp(), $start->getTimestamp());
+    }
+
     public function testTimezoneFail(): void
     {
         $this->expectException(\InvalidArgumentException::class);