fix time conversion bug (#182)

Author: reyang

opentelemetry-sdk/src/opentelemetry/sdk/util.py

@@ -26,5 +26,5 @@ def time_ns():
 
 def ns_to_iso_str(nanoseconds):
     """Get an ISO 8601 string from time_ns value."""
-    ts = datetime.datetime.fromtimestamp(nanoseconds / 1e9)
+    ts = datetime.datetime.utcfromtimestamp(nanoseconds / 1e9)
     return ts.strftime("%Y-%m-%dT%H:%M:%S.%fZ")