Add -Werror=date-time to target builds

The lack of a changing timestamp improves reproducibility of builds, and
helps create smaller OTAs.

Bug: 24204119
Change-Id: I68acfb53be962ffd5c488337517dadb950a13b6f
diff --git a/core/config.mk b/core/config.mk
index 37c781d..da53c34 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -604,7 +604,7 @@
 GLOBAL_CPPFLAGS_NO_OVERRIDE :=
 
 # list of flags to turn specific warnings in to errors
-TARGET_ERROR_FLAGS := -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point
+TARGET_ERROR_FLAGS := -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Werror=date-time
 
 # We run gcc/clang with PWD=/proc/self/cwd to remove the $TOP
 # from the debug output. That way two builds in two different