Index: /applications/editors/josm/plugins/http2/src/org/openstreetmap/josm/plugins/http2/Http2Client.java
===================================================================
--- /applications/editors/josm/plugins/http2/src/org/openstreetmap/josm/plugins/http2/Http2Client.java	(revision 35061)
+++ /applications/editors/josm/plugins/http2/src/org/openstreetmap/josm/plugins/http2/Http2Client.java	(revision 35062)
@@ -179,5 +179,8 @@
         @Override
         public long getLastModified() {
-            return response.headers().firstValueAsLong("Last-Modified").orElse(0L);
+            return response.headers().firstValue("Last-Modified")
+                    .map(DateTimeFormatter.RFC_1123_DATE_TIME::parse)
+                    .map(t -> 1000L * t.getLong(ChronoField.INSTANT_SECONDS))
+                    .orElse(0L);
         }