Introduce a flag to overwrite Date.now() with a fixed value

src/runtime/runtime-date.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/arguments.h"
 #include "src/date.h"
 #include "src/dateparser-inl.h"
 #include "src/messages.h"
@@ skipping 60 matching lines @@
 RUNTIME_FUNCTION(Runtime_DateCurrentTime) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 0);
   if (FLAG_log_timer_events || FLAG_prof_cpp) LOG(isolate, CurrentTimeEvent());
 
   // According to ECMA-262, section 15.9.1, page 117, the precision of
   // the number in a Date object representing a particular instant in
   // time is milliseconds. Therefore, we floor the result of getting
   // the OS time.
   double millis;
-  if (FLAG_verify_predictable) {
+  if (FLAG_fixed_date > 0) {
+    millis = 1388534400000.0;  // Jan 1 2014 00:00:00 GMT+0000
+    millis += 1000.0 * FLAG_fixed_date;
+  } else if (FLAG_verify_predictable) {
     millis = 1388534400000.0;  // Jan 1 2014 00:00:00 GMT+0000
     millis += Floor(isolate->heap()->synthetic_time());
   } else {
     millis = Floor(base::OS::TimeCurrentMillis());
   }
   return *isolate->factory()->NewNumber(millis);
 }
 
 
 RUNTIME_FUNCTION(Runtime_DateParseString) {
@@ skipping 89 matching lines @@
     HandleScope scope(isolate);
     THROW_NEW_ERROR_RETURN_FAILURE(
         isolate, NewTypeError(MessageTemplate::kNotDateObject));
   }
   JSDate* date = JSDate::cast(obj);
   if (index == 0) return date->value();
   return JSDate::GetField(date, Smi::FromInt(index));
 }
 }  // namespace internal
 }  // namespace v8