Emphasizes that TimeTicks::UnixEpoch() is only an estimate

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 coordinated universal time (UTC),
 // internally represented as microseconds (s/1,000,000) since the Windows epoch
 // (1601-01-01 00:00:00 UTC). System-dependent clock interface routines are
 // defined in time_PLATFORM.cc. Note that values for Time may skew and jump
 // around as the operating system makes adjustments to synchronize (e.g., with
 // NTP servers). Thus, client code that uses the Time class must account for
@@ skipping 667 matching lines @@
   // clock will be used instead.
   static bool IsHighResolution();
 
 #if defined(OS_WIN)
   // Translates an absolute QPC timestamp into a TimeTicks value. The returned
   // value has the same origin as Now(). Do NOT attempt to use this if
   // IsHighResolution() returns false.
   static TimeTicks FromQPCValue(LONGLONG qpc_value);
 #endif
 
-  // Get the TimeTick value at the time of the UnixEpoch. This is useful when
-  // you need to relate the value of TimeTicks to a real time and date.
-  // Note: Upon first invocation, this function takes a snapshot of the realtime
-  // clock to establish a reference point.  This function will return the same
-  // value for the duration of the application, but will be different in future
-  // application runs.
+  // Get an estimate of the TimeTick value at the time of the UnixEpoch. Because
+  // Time and TimeTicks respond differently to user-set time and NTP
+  // adjustments, this number is only an estimate. Nevertheless, this can be
+  // useful when you need to relate the value of TimeTicks to a real time and
+  // date. Note: Upon first invocation, this function takes a snapshot of the
+  // realtime clock to establish a reference point.  This function will return
+  // the same value for the duration of the application, but will be different
+  // in future application runs.
   static TimeTicks UnixEpoch();
 
   // Returns |this| snapped to the next tick, given a |tick_phase| and
   // repeating |tick_interval| in both directions. |this| may be before,
   // after, or equal to the |tick_phase|.
   TimeTicks SnappedToNextTick(TimeTicks tick_phase,
                               TimeDelta tick_interval) const;
 
 #if defined(OS_WIN)
  protected:
@@ skipping 93 matching lines @@
   explicit TraceTicks(int64 us) : TimeBase(us) {
   }
 };
 
 // For logging use only.
 BASE_EXPORT std::ostream& operator<<(std::ostream& os, TraceTicks time_ticks);
 
 }  // namespace base
 
 #endif  // BASE_TIME_TIME_H_