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..de1b443bda1db100d4f1ace66cd236a20c03a6c7 100755
--- a/build/write_build_date_header.py
+++ b/build/write_build_date_header.py
@@ -4,40 +4,50 @@
 # 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).
+build_type impacts the timestamp generated:
+- default: the build date is set to the most recent first Sunday of a month at
+  5:00am. The reason is that it is a time where invalidating the build cache
+  shouldn't have major reprecussions (due to lower load).
+- official: the build date is set to the current date at 5:00am, or the day
+  before if the current time is before 5:00am.
+Either way, it is guaranteed to be in the past and always in UTC.
 
 It is also possible to explicitly set a build date to be used.
-
-The reason for using the first Sunday of a month for unofficial builds is that
-it is a time where invalidating the build cache shouldn't have major
-reprecussions (due to lower load).
 """
 
 import argparse
 import calendar
 import datetime
+import doctest
 import os
 import sys
 
 
 def GetFirstSundayOfMonth(year, month):
-  """Returns the first sunday of the given month of the given year."""
+  """Returns the first sunday of the given month of the given year.
+
+  >>> GetFirstSundayOfMonth(2016, 2)
+  7
+  >>> GetFirstSundayOfMonth(2016, 3)
+  6
+  >>> GetFirstSundayOfMonth(2000, 1)
+  2
+  """
   weeks = calendar.Calendar().monthdays2calendar(year, month)
   # Return the first day in the first week that is a Sunday.
   return [date_day[0] for date_day in weeks[0] if date_day[1] == 6][0]
 
 
-# Validate that GetFirstSundayOfMonth works.
-assert GetFirstSundayOfMonth(2016, 2) == 7
-assert GetFirstSundayOfMonth(2016, 3) == 6
-assert GetFirstSundayOfMonth(2000, 1) == 2
-
-
 def GetBuildDate(build_type, utc_now):
-  """Gets the approximate build date given the specific build type."""
+  """Gets the approximate build date given the specific build type.
+
+  >>> GetBuildDate('default', datetime.datetime(2016, 2, 6, 1, 2, 3))
+  'Jan 03 2016 01:02:03'
+  >>> GetBuildDate('default', datetime.datetime(2016, 2, 7, 5))
+  'Feb 07 2016 05:00:00'
+  >>> GetBuildDate('default', datetime.datetime(2016, 2, 8, 5))
+  'Feb 07 2016 05:00:00'
+  """
   day = utc_now.day
   month = utc_now.month
   year = utc_now.year
@@ -54,28 +64,39 @@ def GetBuildDate(build_type, utc_now):
         month = 12
         year -= 1
       day = GetFirstSundayOfMonth(year, month)
-  return '{:%b %d %Y}'.format(datetime.date(year, month, day))
-
-
-# Validate that GetBuildDate works.
-assert GetBuildDate('default', datetime.date(2016, 2, 6)) == 'Jan 03 2016'
-assert GetBuildDate('default', datetime.date(2016, 2, 7)) == 'Feb 07 2016'
-assert GetBuildDate('default', datetime.date(2016, 2, 8)) == 'Feb 07 2016'
+  now = datetime.datetime(
+      year, month, day, utc_now.hour, utc_now.minute, utc_now.second)
+  return '{:%b %d %Y %H:%M:%S}'.format(now)
 
 
 def main():
-  argument_parser = argparse.ArgumentParser()
+  if doctest.testmod()[0]:
+    return 1
+  argument_parser = argparse.ArgumentParser(
+      description=sys.modules[__name__].__doc__,
+      formatter_class=argparse.RawDescriptionHelpFormatter)
   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')
+  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. Format must be '
+           '\'Mmm DD YYYY HH:MM:SS\'')
   args = argument_parser.parse_args()
 
   if args.build_date_override:
+    # Format is expected to be "Mmm DD YYYY HH:MM:SS".
     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. Use the same calculation
+      # as the day before.
+      # See //base/build_time.cc.
+      now = now - datetime.timedelta(day=1)
+    now = datetime.datetime(now.year, now.month, now.day, 5, 0, 0)
+    build_date = GetBuildDate(args.build_type, now)
 
   output = ('// Generated by //build/write_build_date_header.py\n'
            '#ifndef BUILD_DATE\n'