On review fixed an issue with how aliases are resolved

src/main/java/net/fortuna/ical4j/model/TimeZoneRegistry.java

@@ -66,7 +66,7 @@ static ZoneId getGlobalZoneId(String tzId) {
         ZoneId zoneId = ZoneId.of(tzId, ZONE_ALIASES);
         Optional<Map.Entry<String, String>> lookup = ZONE_IDS.entrySet().stream().filter(entry ->
                 entry.getValue().equals(zoneId.getId())).findFirst();
-        return lookup.map(mapping -> ZoneId.of(mapping.getKey())).orElseThrow();
+        return lookup.map(mapping -> ZoneId.of(mapping.getKey())).orElse(zoneId);
     }
 
     /**

src/test/groovy/net/fortuna/ical4j/model/TimeZoneRegistryTest.groovy

@@ -0,0 +1,49 @@
+/*
+ *  Copyright (c) 2024, Ben Fortuna
+ *  All rights reserved.
+ *
+ *  Redistribution and use in source and binary forms, with or without
+ *  modification, are permitted provided that the following conditions
+ *  are met:
+ *
+ *   o Redistributions of source code must retain the above copyright
+ *  notice, this list of conditions and the following disclaimer.
+ *
+ *   o Redistributions in binary form must reproduce the above copyright
+ *  notice, this list of conditions and the following disclaimer in the
+ *  documentation and/or other materials provided with the distribution.
+ *
+ *   o Neither the name of Ben Fortuna nor the names of any other contributors
+ *  may be used to endorse or promote products derived from this software
+ *  without specific prior written permission.
+ *
+ *  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ *  "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ *  LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ *  A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ *  CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ *  EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ *  PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ *  PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ *  LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+ *  NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ *  SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+package net.fortuna.ical4j.model
+
+import spock.lang.Specification
+
+class TimeZoneRegistryTest extends Specification {
+
+    def 'test global zone id retrieval'() {
+        expect: 'matching result'
+        TimeZoneRegistry.getGlobalZoneId(id) == TimeZoneRegistry.getGlobalZoneId(expectedZoneId)
+
+        where:
+        id              | expectedZoneId
+        'UTC'           | 'Etc/UTC'
+        'Europe/London' | 'Europe/London'
+    }
+}

src/test/groovy/net/fortuna/ical4j/model/TimeZoneSpec.groovy

@@ -43,6 +43,7 @@ import spock.lang.Unroll
 import java.time.LocalDate
 import java.time.ZoneId
 import java.time.ZoneOffset
+import java.util.stream.Collectors
 
 @Slf4j
 class TimeZoneSpec extends Specification {
@@ -217,6 +218,9 @@ class TimeZoneSpec extends Specification {
 		System.clearProperty('net.fortuna.ical4j.timezone.update.enabled')
 
 		where:
-		alias << ZoneId.getAvailableZoneIds()
+		alias << getClass().getResourceAsStream('/net/fortuna/ical4j/model/tz.alias').readLines().stream()
+				.filter(line -> !line.empty && !line.contains('#')).map {
+			line -> line.split('\\s*=\\s*')[0]
+		}.collect(Collectors.toList())
 	}
 }