diff --git a/core/version_defaults.mk b/core/version_defaults.mk
index 9f76bc8..858384b 100644
--- a/core/version_defaults.mk
+++ b/core/version_defaults.mk
@@ -254,12 +254,23 @@
   BUILD_ID := UNKNOWN
 endif
 
-ifneq (,$(findstring Darwin,$(UNAME)))
-DATE := date -r $(shell cat $(BUILD_DATETIME_FILE))
-else
-DATE := date -d @$(shell cat $(BUILD_DATETIME_FILE))
+ifndef BUILD_DATETIME
+  # Used to reproduce builds by setting the same time. Must be the number
+  # of seconds since the Epoch.
+  BUILD_DATETIME := $(shell date +%s)
 endif
 
+ifneq (,$(findstring Darwin,$(UNAME)))
+DATE := date -r $(BUILD_DATETIME)
+else
+DATE := date -d @$(BUILD_DATETIME)
+endif
+
+# Everything should be using BUILD_DATETIME_FROM_FILE instead.
+# BUILD_DATETIME and DATE can be removed once BUILD_NUMBER moves
+# to soong_ui.
+BUILD_DATETIME :=
+
 ifndef BUILD_NUMBER
   # BUILD_NUMBER should be set to the source control value that
   # represents the current state of the source code.  E.g., a