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

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index 441ba102c521c3f69b9a8c5ba3c58abf03908d02..cc1d2e8b239bc7678d798b0965cb591043609a5a 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -28,7 +28,8 @@ 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"
 }