Document more explicitly that Time lives in UTC.

base/time/time.h

 // Copyright (c) 2012 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.
 
-// Time represents an absolute point in time, internally represented as
-// microseconds (s/1,000,000) since the Windows epoch (1601-01-01 00:00:00 UTC)
-// (See http://crbug.com/14734).  System-dependent clock interface routines are
-// defined in time_PLATFORM.cc.
+// Time represents an absolute point in coordinated universal time (UTC),
+// internally represented as microseconds (s/1,000,000) since the Windows epoch
+// (1601-01-01 00:00:00 UTC) (See http://crbug.com/14734).  System-dependent
+// clock interface routines are defined in time_PLATFORM.cc.
 //
 // TimeDelta represents a duration of time, internally represented in
 // microseconds.
 //
 // TimeTicks represents an abstract time that is most of the time incrementing
 // for use in measuring time durations. It is internally represented in
 // microseconds.  It can not be converted to a human-readable time, but is
 // guaranteed not to decrease (if the user changes the computer clock,
 // Time::Now() may actually decrease or jump).  But note that TimeTicks may
 // "stand still", for example if the computer suspended.
@@ skipping 182 matching lines @@
   // Delta in microseconds.
   int64 delta_;
 };
 
 inline TimeDelta operator*(int64 a, TimeDelta td) {
   return TimeDelta(a * td.delta_);
 }
 
 // Time -----------------------------------------------------------------------
 
-// Represents a wall clock time.
+// Represents a wall clock time in UTC.
 class BASE_EXPORT Time {
  public:
   static const int64 kMillisecondsPerSecond = 1000;
   static const int64 kMicrosecondsPerMillisecond = 1000;
   static const int64 kMicrosecondsPerSecond = kMicrosecondsPerMillisecond *
                                               kMillisecondsPerSecond;
   static const int64 kMicrosecondsPerMinute = kMicrosecondsPerSecond * 60;
   static const int64 kMicrosecondsPerHour = kMicrosecondsPerMinute * 60;
   static const int64 kMicrosecondsPerDay = kMicrosecondsPerHour * 24;
   static const int64 kMicrosecondsPerWeek = kMicrosecondsPerDay * 7;
@@ skipping 508 matching lines @@
 #endif
 };
 
 inline TimeTicks TimeDelta::operator+(TimeTicks t) const {
   return TimeTicks(t.ticks_ + delta_);
 }
 
 }  // namespace base
 
 #endif  // BASE_TIME_TIME_H_