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

build/write_build_date_header.py

 #!/usr/bin/env python
 # Copyright (c) 2016 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.
 """Writes a file that contains a define that approximates the build date.
 
 For unofficial builds, the build date is set to the most recent first Sunday
 of a month, in UTC time.
 
 For official builds, the build date is set to the current date (in UTC).
@@ skipping 57 matching lines @@
   argument_parser.add_argument('output_file', help='The file to write to')
   argument_parser.add_argument('build_type', help='The type of build',
                                choices=('official', 'default'))
   argument_parser.add_argument('build_date_override', nargs='?',
                                help='Optional override for the build date')
   args = argument_parser.parse_args()
 
   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'
            '#define BUILD_DATE "{}"\n'
            '#endif // BUILD_DATE\n'.format(build_date))
 
   current_contents = ''
   if os.path.isfile(args.output_file):
     with open(args.output_file, 'r') as current_file:
       current_contents = current_file.read()
 
   if current_contents != output:
     with open(args.output_file, 'w') as output_file:
       output_file.write(output)
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main())