Improve the error message in BuildTime.InThePast to help figure out the problem.

base/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # HOW TO WRITE CONDITIONALS IN THIS FILE
 # ======================================
 #
 # In many other places, one would write a conditional that expresses all the
 # cases when a source file is used or unused, and then either add or subtract
 # it from the sources list in that case
@@ skipping 10 matching lines @@
 import("//build/config/allocator.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/config/sysroot.gni")
 import("//build/config/ui.gni")
 import("//build/nocompile.gni")
 import("//testing/test.gni")
 
 declare_args() {
   # Override this value to give a specific build date.
-  # See //base/build_time.cc for more details.
+  # See //base/build_time.cc and //build/write_build_date_header.py for more
+  # details and the expected format.
   override_build_date = "N/A"
 }
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
 if (is_win) {
   import("//build/config/win/visual_studio_version.gni")
 }
@@ skipping 2282 matching lines @@
 
   # GYP: //base.gyp:base_java_unittest_support
   android_library("base_java_unittest_support") {
     deps = [
       ":base_java",
     ]
     java_files =
         [ "test/android/java/src/org/chromium/base/ContentUriTestUtils.java" ]
   }
 }