Update the timestamp to default to UTC. (tensorflow#4040)

Previously the timestamp we pushed to bigquery was PDT, but the
timezone spec was set to 'Z' (UTC). This is provide incorrect
value to bigquery, and might affect analysis down the road.

official/utils/logs/logger.py

@@ -149,7 +149,7 @@ def log_metric(self, name, value, unit=None, global_step=None, extras=None):
           "value": float(value),
           "unit": unit,
           "global_step": global_step,
-          "timestamp": datetime.datetime.now().strftime(
+          "timestamp": datetime.datetime.utcnow().strftime(
               _DATE_TIME_FORMAT_PATTERN),
           "extras": extras}
       try:
@@ -184,7 +184,8 @@ def _gather_run_info(model_name):
   run_info = {
       "model_name": model_name,
       "machine_config": {},
-      "run_date": datetime.datetime.now().strftime(_DATE_TIME_FORMAT_PATTERN)}
+      "run_date": datetime.datetime.utcnow().strftime(
+          _DATE_TIME_FORMAT_PATTERN)}
   _collect_tensorflow_info(run_info)
   _collect_tensorflow_environment_variables(run_info)
   _collect_cpu_info(run_info)