@@ -1410,6 +1410,9 @@ config("features") {
   if (v8_enable_temporal_support) {
     defines += [ "V8_TEMPORAL_SUPPORT" ]
   }
+  if (v8_enable_temporal_systemicu) {
+    defines += [ "V8_ENABLE_TEMPORAL_SYSTEMICU" ]
+  }
   if (v8_enable_local_handle_zapping) {
     defines += [ "ENABLE_LOCAL_HANDLE_ZAPPING" ]
   }
@@ -3248,6 +3251,7 @@ generated_file("v8_generate_features_json") {
     v8_enable_hugepage = v8_enable_hugepage
     v8_enable_i18n_support = v8_enable_i18n_support
     v8_enable_temporal_support = v8_enable_temporal_support
+    v8_enable_temporal_systemicu = v8_enable_temporal_systemicu
     v8_enable_javascript_promise_hooks = v8_enable_javascript_promise_hooks
     v8_enable_lite_mode = v8_enable_lite_mode
     v8_enable_map_packing = v8_enable_map_packing
 
@@ -63,6 +63,10 @@ declare_args() {
   # Furthermore, some architectures don't have Rust toolchains in Chromium
   v8_enable_temporal_support = !(defined(build_with_node) && build_with_node)
 
+  # by default, don't enable compiling Temporal with a system ICU4C
+  # (assume private headers are available)
+  v8_enable_temporal_systemicu = false
+
   # Use static libraries instead of source_sets.
   v8_static_library = false
 
 
@@ -11,8 +11,36 @@
 #include "temporal_rs/TimeZone.hpp"
 
 #ifdef V8_INTL_SUPPORT
+#ifndef V8_ENABLE_TEMPORAL_SYSTEMICU
+// ICU internal header file
 #include "udatamem.h"
 #else
+/**
+ * Shadow definition of UDataMemory
+ * This is functionally identical to the definition in udatamem.h
+ * Irrelevant fields are marked 'ignored' and should not be used.
+ *
+ * This definition is copied here so that the result of udata_open()
+ * can be used and length-checked without needing to resort to
+ * calling internal ICU functions.
+ *
+ * Note, there is an ICU ticket,
+ * https://unicode-org.atlassian.net/browse/ICU-23400
+ * to consider whether the API surface should be changed
+ * here. If and when this is done, such an API could be used.
+ */
+struct UDataMemoryShadow {
+  const void* ignored1;
+  const void* data;  //< pointer to header of data object
+  const void* ignored2;
+  UBool ignored3;
+  void* ignored4;
+  void* ignored5;
+  int32_t
+      length;  //< length of entire region pointed to by data if known, else -1
+};
+#endif
+#else
 // Defined in builtins-temporal-zoneinfo64-data.cc, generated by
 // include-file-as-bytes.py
 extern "C" uint32_t zoneinfo64_static_data[];
@@ -30,17 +58,29 @@ ZoneInfo64Provider::ZoneInfo64Provider() {
     provider = temporal_rs::Provider::empty();
     return;
   }
+#ifndef V8_ENABLE_TEMPORAL_SYSTEMICU
   // NOT udata_getLength: this ignores the header,
   // and we're parsing resb files with the header
   auto length = memory->length;
   const void* data = udata_getRawMemory(memory);
-  DCHECK_WITH_MSG(length % 4 == 0, "ICU4C should align udata to uint32_t");
-  if (length % 4 != 0) {
-    // This really shouldn't happen: ICU4C aligns these files
-    // to 4 when baking them in
+#else
+  // reinterpret with a local struct
+  const UDataMemoryShadow* shadowMemory =
+      reinterpret_cast<const UDataMemoryShadow*>(memory);
+  // just need the length and data
+  auto length = shadowMemory->length;
+  auto data = shadowMemory->data;
+#endif
+  // This really shouldn't happen: ICU4C pads these files
+  // to 4 when baking them in
+  DCHECK_WITH_MSG(length % 4 == 0, "ICU4C should pad udata to uint32_t");
+  // Length may not be known. If we're not OK with that, don't proceed.
+  DCHECK_WITH_MSG(length != -1, "ICU4C usually knows the length");
+  if (length % 4 != 0 || length == -1) {
     provider = temporal_rs::Provider::empty();
     return;
   }
+  DCHECK_WITH_MSG(data != nullptr, "ICU4C returned nullptr");
 
   const uint32_t* data_32 = static_cast<const uint32_t*>(data);
   std::span<const uint32_t> data_span(data_32, length / 4);