diff --git a/core/main.mk b/core/main.mk
index 0c165ca..ef55b4e 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -61,12 +61,23 @@
 # when using ninja.
 $(shell mkdir -p $(OUT_DIR) && \
     echo -n $(BUILD_NUMBER) > $(OUT_DIR)/build_number.txt)
+BUILD_NUMBER_FILE := $(OUT_DIR)/build_number.txt
+
 ifeq ($(HOST_OS),darwin)
 DATE_FROM_FILE := date -r $(BUILD_DATETIME_FROM_FILE)
 else
 DATE_FROM_FILE := date -d @$(BUILD_DATETIME_FROM_FILE)
 endif
 
+# Pick a reasonable string to use to identify files.
+ifeq ($(strip $(HAS_BUILD_NUMBER)),false)
+  # BUILD_NUMBER has a timestamp in it, which means that
+  # it will change every time.  Pick a stable value.
+  FILE_NAME_TAG := eng.$(USER)
+else
+  FILE_NAME_TAG := $(file <$(BUILD_NUMBER_FILE))
+endif
+
 # Make an empty directory, which can be used to make empty jars
 EMPTY_DIRECTORY := $(OUT_DIR)/empty
 $(shell mkdir -p $(EMPTY_DIRECTORY) && rm -rf $(EMPTY_DIRECTORY)/*)