Index: clang/test/Driver/check-time-trace-sections.py
===================================================================
--- clang/test/Driver/check-time-trace-sections.py
+++ clang/test/Driver/check-time-trace-sections.py
@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 
-import json, sys
+import json, sys, time
 
 def is_inside(range1, range2):
     a = range1["ts"]; b = a + range1["dur"]
@@ -11,11 +11,20 @@
     b = range1["ts"] + range1["dur"]; c = range2["ts"]
     return b <= c
 
-events = json.loads(sys.stdin.read())["traceEvents"]
+log_contents = json.loads(sys.stdin.read())
+events = log_contents["traceEvents"]
 codegens = [event for event in events if event["name"] == "CodeGen Function"]
 frontends = [event for event in events if event["name"] == "Frontend"]
 backends = [event for event in events if event["name"] == "Backend"]
 
+beginning_of_time = log_contents["beginningOfTime"] / 1000000
+seconds_since_epoch = time.time()
+
+# Make sure that the 'beginningOfTime' is not earlier than 10 seconds ago.
+if seconds_since_epoch - beginning_of_time > 10:
+    sys.exit("'beginningOfTime' should represent the absolute time when the "
+             "process has started")
+
 if not all([any([is_inside(codegen, frontend) for frontend in frontends])
                         for codegen in codegens]):
     sys.exit("Not all CodeGen sections are inside any Frontend section!")
Index: clang/test/Driver/check-time-trace.cpp
===================================================================
--- clang/test/Driver/check-time-trace.cpp
+++ clang/test/Driver/check-time-trace.cpp
@@ -3,7 +3,8 @@
 // RUN:   | %python -c 'import json, sys; json.dump(json.loads(sys.stdin.read()), sys.stdout, sort_keys=True, indent=2)' \
 // RUN:   | FileCheck %s
 
-// CHECK: "traceEvents": [
+// CHECK: "beginningOfTime": {{[0-9]{16},}}
+// CHECK-NEXT: "traceEvents": [
 // CHECK: "args":
 // CHECK: "detail":
 // CHECK: "dur":
Index: lld/test/ELF/time-trace.s
===================================================================
--- lld/test/ELF/time-trace.s
+++ lld/test/ELF/time-trace.s
@@ -18,7 +18,8 @@
 # RUN:   | %python -c 'import json, sys; json.dump(json.loads(sys.stdin.read()), sys.stdout, sort_keys=True, indent=2)' \
 # RUN:   | FileCheck %s
 
-# CHECK: "traceEvents": [
+# CHECK: "beginningOfTime": {{[0-9]{16},}}
+# CHECK-NEXT: "traceEvents": [
 
 # Check one event has correct fields
 # CHECK:      "dur":
Index: llvm/lib/Support/TimeProfiler.cpp
===================================================================
--- llvm/lib/Support/TimeProfiler.cpp
+++ llvm/lib/Support/TimeProfiler.cpp
@@ -251,6 +251,21 @@
 
     J.arrayEnd();
     J.attributeEnd();
+
+    // Emit synchronization point, i. e. the absolute time of StartTime.
+    // When combining time profiler logs from different processes,
+    // this attribute helps preserve relative timing.
+    {
+      const auto SystemTime =
+          time_point_cast<microseconds>(system_clock::now());
+      const microseconds ProcessLocalTime =
+          time_point_cast<microseconds>(steady_clock::now()) -
+          time_point_cast<microseconds>(StartTime);
+      const auto BeginningOfTimeUs = SystemTime - ProcessLocalTime;
+      J.attribute("beginningOfTime",
+                  BeginningOfTimeUs.time_since_epoch().count());
+    }
+
     J.objectEnd();
   }