diff --git a/java/kotlin-extractor/src/main/java/com/semmle/extractor/java/OdasaOutput.java b/java/kotlin-extractor/src/main/java/com/semmle/extractor/java/OdasaOutput.java
index 01861bf5ca6..0da4b0acda1 100644
--- a/java/kotlin-extractor/src/main/java/com/semmle/extractor/java/OdasaOutput.java
+++ b/java/kotlin-extractor/src/main/java/com/semmle/extractor/java/OdasaOutput.java
@@ -4,10 +4,14 @@ import java.lang.reflect.*;
 import java.io.File;
 import java.io.IOException;
 import java.util.Arrays;
+import java.util.Enumeration;
+import java.util.HashMap;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.Objects;
 import java.util.regex.Pattern;
+import java.util.zip.ZipEntry;
+import java.util.zip.ZipFile;
 
 import com.github.codeql.Logger;
 import static com.github.codeql.ClassNamesKt.getIrDeclBinaryName;
@@ -547,6 +551,49 @@ public class OdasaOutput {
 					(tcv.majorVersion == majorVersion && tcv.minorVersion == minorVersion &&
 					tcv.lastModified < lastModified);
 		}
+
+        private static Map<String, Map<String, Long>> jarFileEntryTimeStamps = new HashMap<>();
+
+        private static Map<String, Long> getZipFileEntryTimeStamps(String path, Logger log) {
+            try {
+                Map<String, Long> result = new HashMap<>();
+                ZipFile zf = new ZipFile(path);
+                Enumeration<? extends ZipEntry> entries = zf.entries();
+                while (entries.hasMoreElements()) {
+                    ZipEntry ze = entries.nextElement();
+                    result.put(ze.getName(), ze.getLastModifiedTime().toMillis());
+                }
+                return result;
+            } catch(IOException e) {
+                log.warn("Failed to get entry timestamps from " + path, e);
+                return null;
+            }
+        }
+
+        private static long getVirtualFileTimeStamp(VirtualFile vf, Logger log) {
+            if (vf.getFileSystem().getProtocol().equals("jar")) {
+                String[] parts = vf.getPath().split("!/");
+                if (parts.length == 2) {
+                    String jarFilePath = parts[0];
+                    String entryPath = parts[1];
+                    if (!jarFileEntryTimeStamps.containsKey(jarFilePath)) {
+                        jarFileEntryTimeStamps.put(jarFilePath, getZipFileEntryTimeStamps(jarFilePath, log));
+                    }
+                    Map<String, Long> entryTimeStamps = jarFileEntryTimeStamps.get(jarFilePath);
+                    if (entryTimeStamps != null) {
+                        Long entryTimeStamp = entryTimeStamps.get(entryPath);
+                        if (entryTimeStamp != null)
+                            return entryTimeStamp;
+                        else
+                            log.warn("Couldn't find timestamp for jar file " + jarFilePath + " entry " + entryPath);
+                    }
+                }
+            }
+
+            // For all files except for jar files, and a fallback in case of I/O problems reading a jar file:
+            return vf.getTimeStamp();
+        }
+
 		private static TrapClassVersion fromSymbol(IrDeclaration sym, Logger log) {
 			VirtualFile vf = sym instanceof IrClass ? getIrClassVirtualFile((IrClass)sym) :
 					sym.getParent() instanceof IrClass ? getIrClassVirtualFile((IrClass)sym.getParent()) :
@@ -583,7 +630,7 @@ public class OdasaOutput {
 				};
 				(new ClassReader(vf.contentsToByteArray())).accept(versionGetter, ClassReader.SKIP_CODE | ClassReader.SKIP_DEBUG | ClassReader.SKIP_FRAMES);
 
-				return new TrapClassVersion(versionStore[0] & 0xffff, versionStore[0] >> 16, vf.getTimeStamp(), "kotlin");
+				return new TrapClassVersion(versionStore[0] & 0xffff, versionStore[0] >> 16, getVirtualFileTimeStamp(vf, log), "kotlin");
 			}
 			catch(IllegalAccessException e) {
 				log.warn("Failed to read class file version information", e);