- woops, BeginningOfTime must always capture now according to the system clock, irrespective of what we use for the profiling clock.

llvm/lib/Support/TimeProfiler.cpp

 using namespace llvm;
 
 namespace {
 
 using std::chrono::duration;
 using std::chrono::duration_cast;
 using std::chrono::microseconds;
 using std::chrono::steady_clock;
+using std::chrono::system_clock;
 using std::chrono::time_point;
 using std::chrono::time_point_cast;
 
 struct TimeTraceProfilerInstances {
   std::mutex Lock;
   std::vector<TimeTraceProfiler *> List;
 };
 
return (time_point_cast<microseconds>(End) -
         .count();
   }
 };
 
 } // anonymous namespace
 
 struct llvm::TimeTraceProfiler {
   TimeTraceProfiler(unsigned TimeTraceGranularity = 0, StringRef ProcName = "")
-      : BeginningOfTime(ClockType::now()), StartTime(ClockType::now()),
+      : BeginningOfTime(system_clock::now()), StartTime(ClockType::now()),
         ProcName(ProcName), Pid(sys::Process::getProcessId()),
         Tid(llvm::get_threadid()), TimeTraceGranularity(TimeTraceGranularity) {
     llvm::get_thread_name(ThreadName);
   }
 
   void begin(TimeTraceProfilerEntry &&Entry) { Stack.emplace_back(Entry); }
 
   void end() {
J.attribute("beginningOfTime",
                     .count());
 
     J.objectEnd();
   }
 
   SmallVector<TimeTraceProfilerEntry, 16> Stack;
   SmallVector<TimeTraceProfilerEntry, 128> Entries;
   StringMap<CountAndDurationType> CountAndTotalPerName;
-  const TimePointType BeginningOfTime;
+  // System clock time when the session was begun.
+  const time_point<system_clock> BeginningOfTime;
+  // Profiling clock time when the session was begun.
   const TimePointType StartTime;
   const std::string ProcName;
   const sys::Process::Pid Pid;
   SmallString<0> ThreadName;
   const uint64_t Tid;
 
   // Minimum time granularity (in microseconds)
   const unsigned TimeTraceGranularity;