Error Prone: Enable DateFormatConstant at ERROR severity

Change-Id: Ie9e3e17574d5ee6bd7b25efc59f0bc17dde0b935

tools/BUILD

diff --git a/tools/BUILD b/tools/BUILD
index bc8e0da..9294fcf 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -29,6 +29,7 @@
         "-Xep:CannotMockFinalClass:ERROR",
         "-Xep:ClassCanBeStatic:ERROR",
         "-Xep:ClassNewInstance:ERROR",
+        "-Xep:DateFormatConstant:ERROR",
         "-Xep:DefaultCharset:ERROR",
         "-Xep:DoubleCheckedLocking:ERROR",
         "-Xep:DoubleCheckedLocking:ERROR",