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

build/write_build_date_header.py

Index: build/write_build_date_header.py
diff --git a/build/write_build_date_header.py b/build/write_build_date_header.py
index 0ba47049b570377c4e090a4c9d1fc7884afb9261..37188950c91782f0792d53b7730857c5b99eb223 100755
--- a/build/write_build_date_header.py
+++ b/build/write_build_date_header.py
@@ -75,7 +75,13 @@ def main():
   if args.build_date_override:
     build_date = args.build_date_override
   else:
-    build_date = GetBuildDate(args.build_type, datetime.datetime.utcnow())
+    now = datetime.datetime.utcnow()
+    if now.hour < 5:
+      # The time is locked at 5:00 am in UTC to cause the build cache
+      # invalidation to not happen exactly at midnight. Take the day before.
+      # See //base/build_time.cc.

[Nico, 2016/03/30 20:34:00]

This should also refer to the script that has the constant 5 … and the other
script should have a comment pointing to this one.

[M-A Ruel, 2016/03/30 20:38:09]

Humm what about changing the script to output the hour in the constant? I guess
I'll just do that. 

+      now = now - datetime.timedelta(day=1)
+    build_date = GetBuildDate(args.build_type, now)
 
   output = ('// Generated by //build/write_build_date_header.py\n'
            '#ifndef BUILD_DATE\n'