Revert "Use repeatable timestamps"

OSX `date` doesn't understand -d

This reverts commit 13ba6e00f4c7e04f1038e94ab98e3ed424daa4fa.

Change-Id: I5970032ff02cc07bd3b087e1eeb5f19b1e4194b4
diff --git a/core/version_defaults.mk b/core/version_defaults.mk
index 9b2ecf2..f3b2297 100644
--- a/core/version_defaults.mk
+++ b/core/version_defaults.mk
@@ -24,7 +24,6 @@
 #     DEFAULT_APP_TARGET_SDK
 #     BUILD_ID
 #     BUILD_NUMBER
-#     BUILD_DATETIME
 #
 
 # Look for an optional file containing overrides of the defaults,
@@ -90,12 +89,6 @@
   BUILD_ID := UNKNOWN
 endif
 
-ifeq "" "$(BUILD_DATETIME)"
-  # Used to reproduce builds by setting the same time. Must be in a
-  # format understood by `date`
-  BUILD_DATETIME := $(shell date)
-endif
-
 ifeq "" "$(BUILD_NUMBER)"
   # BUILD_NUMBER should be set to the source control value that
   # represents the current state of the source code.  E.g., a
@@ -106,5 +99,5 @@
   # If no BUILD_NUMBER is set, create a useful "I am an engineering build
   # from this date/time" value.  Make it start with a non-digit so that
   # anyone trying to parse it as an integer will probably get "0".
-  BUILD_NUMBER := eng.$(USER).$(shell date -d "$(BUILD_DATETIME)" +%Y%m%d.%H%M%S)
+  BUILD_NUMBER := eng.$(USER).$(shell date +%Y%m%d.%H%M%S)
 endif