diff --git a/zipline/data/loader.py b/zipline/data/loader.py
index 2a0b91bd..a6ce5b7c 100644
--- a/zipline/data/loader.py
+++ b/zipline/data/loader.py
@@ -162,10 +162,12 @@ Fetching data from Yahoo Finance.
     # we have data,then we need to update
     if len(trading_days) - last_bm_date_offset > 1:
         benchmark_returns = update_benchmarks(bm_symbol, last_bm_date)
-        benchmark_returns = benchmark_returns.tz_localize('UTC')
+        if benchmark_returns.index.tz.zone != 'UTC':
+            benchmark_returns = benchmark_returns.tz_localize('UTC')
     else:
         benchmark_returns = saved_benchmarks
-        benchmark_returns = benchmark_returns.tz_localize('UTC')
+        if benchmark_returns.index.tz.zone != 'UTC':
+            benchmark_returns = benchmark_returns.tz_localize('UTC')
 
     try:
         fp_tr = get_datafile('treasury_curves.csv', "rb")