diff --git a/writer/HDF5DataFile.cpp b/writer/HDF5DataFile.cpp
index c38a54a0..8f0be2d7 100644
--- a/writer/HDF5DataFile.cpp
+++ b/writer/HDF5DataFile.cpp
@@ -16,6 +16,7 @@
 #include "HDF5NXmx.h"
 #include "HDF5DataFilePluginImageStats.h"
 #include "HDF5DataFilePluginReflection.h"
+#include "../common/time_utc.h"
 
 HDF5DataFile::HDF5DataFile(const StartMessage &msg, uint64_t in_file_number) {
     file_number = in_file_number;
@@ -30,7 +31,7 @@ HDF5DataFile::HDF5DataFile(const StartMessage &msg, uint64_t in_file_number) {
     filename = HDF5Metadata::DataFileName(msg, file_number);
     image_low = file_number * msg.images_per_file;
 
-    tmp_filename = fmt::format("{}.{:x}.tmp", filename, std::chrono::system_clock::now().time_since_epoch().count());
+    tmp_filename = fmt::format("{}.{:08x}.tmp", filename, parse_UTC_to_ms(msg.arm_date));
     plugins.emplace_back(std::make_unique<HDF5DataFilePluginROI>());
     plugins.emplace_back(std::make_unique<HDF5DataFilePluginDetector>());
     plugins.emplace_back(std::make_unique<HDF5DataFilePluginAzInt>(msg));
diff --git a/writer/HDF5NXmx.cpp b/writer/HDF5NXmx.cpp
index 4f726a76..f4281216 100644
--- a/writer/HDF5NXmx.cpp
+++ b/writer/HDF5NXmx.cpp
@@ -15,7 +15,7 @@ NXmx::NXmx(const StartMessage &start)
         : start_message(start),
           filename(start.file_prefix + "_master.h5") {
 
-    tmp_filename = fmt::format("{}.{:x}.tmp", filename, std::chrono::system_clock::now().time_since_epoch().count());
+    tmp_filename = fmt::format("{}.{:08x}.tmp", filename, parse_UTC_to_ms(start.arm_date));
 
     if (start.overwrite.has_value())
         overwrite = start.overwrite.value();