review: initialize start timestamp and time

3 files changed, 13 insertions, 3 deletions

diff --git a/src/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.DateTime.corefx.cs b/src/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.DateTime.corefx.cs
index c485b41b7b..ca167d0be9 100644
--- a/src/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.DateTime.corefx.cs
+++ b/src/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.DateTime.corefx.cs
@@ -6,8 +6,12 @@ namespace System.Diagnostics
 {
     partial class Activity
     {
+        /// <summary>
+        /// Returns high resolution (~1 usec) current UTC DateTime.
+        /// </summary>
         private DateTime GetUtcNow()
         {
+            // .NET Core CLR gives accurate UtcNow
             return DateTime.UtcNow;
         }
     }
diff --git a/src/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.DateTime.netfx.cs b/src/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.DateTime.netfx.cs
index d7e7ec349e..dade796b38 100644
--- a/src/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.DateTime.netfx.cs
+++ b/src/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.DateTime.netfx.cs
@@ -8,8 +8,14 @@ namespace System.Diagnostics
 {
     partial class Activity
     {
+        /// <summary>
+        /// Returns high resolution (1 DateTime tick) current UTC DateTime. 
+        /// </summary>
         private DateTime GetUtcNow()
         {
+            // DateTime.UtcNow accuracyon .NET Framework is 16ms, this method 
+            // uses combination of Stopwatch and DateTime to calculate accurate UtcNow.
+
             // Timer ticks need to be converted to DateTime ticks
             long dateTimeTicksDiff = (long)((Stopwatch.GetTimestamp() - syncStopwatchTicks) * 10000000L /
                                    (double)Stopwatch.Frequency);
@@ -26,8 +32,8 @@ namespace System.Diagnostics
             syncUtcNow = DateTime.UtcNow;
         }
 
-        private static DateTime syncUtcNow;
-        private static long syncStopwatchTicks;
+        private static DateTime syncUtcNow = DateTime.UtcNow;
+        private static long syncStopwatchTicks = Stopwatch.GetTimestamp();
 
         // sync DateTime and Stopwatch ticks every 2 hours
         private static Timer syncTimeUpdater = new Timer(s => { Sync(); }, null, 0, 7200000);
diff --git a/src/System.Diagnostics.DiagnosticSource/tests/ActivityDateTimeTests.cs b/src/System.Diagnostics.DiagnosticSource/tests/ActivityDateTimeTests.cs
index feb17e2f13..7aee8cd0f1 100644
--- a/src/System.Diagnostics.DiagnosticSource/tests/ActivityDateTimeTests.cs
+++ b/src/System.Diagnostics.DiagnosticSource/tests/ActivityDateTimeTests.cs
@@ -13,7 +13,7 @@ namespace System.Diagnostics.Tests
             var sw = Stopwatch.StartNew();
 
             activity.Start();
-            SpinWait.SpinUntil(() => sw.ElapsedMilliseconds > 1, 3);
+            SpinWait.SpinUntil(() => sw.ElapsedMilliseconds > 1, 2);
             activity.Stop();
             
             sw.Stop();