diff --git a/superset/db_engine_specs.py b/superset/db_engine_specs.py
index 772dda6a2..7cf4a51d9 100644
--- a/superset/db_engine_specs.py
+++ b/superset/db_engine_specs.py
@@ -485,13 +485,13 @@ class OracleEngineSpec(PostgresBaseEngineSpec):
     time_grain_functions = {
         None: '{col}',
         'PT1S': 'CAST({col} as DATE)',
-        'PT1M': "TRUNC(TO_DATE({col}), 'MI')",
-        'PT1H': "TRUNC(TO_DATE({col}), 'HH')",
-        'P1D': "TRUNC(TO_DATE({col}), 'DDD')",
-        'P1W': "TRUNC(TO_DATE({col}), 'WW')",
-        'P1M': "TRUNC(TO_DATE({col}), 'MONTH')",
-        'P0.25Y': "TRUNC(TO_DATE({col}), 'Q')",
-        'P1Y': "TRUNC(TO_DATE({col}), 'YEAR')",
+        'PT1M': "TRUNC(CAST({col} as DATE), 'MI')",
+        'PT1H': "TRUNC(CAST({col} as DATE), 'HH')",
+        'P1D': "TRUNC(CAST({col} as DATE), 'DDD')",
+        'P1W': "TRUNC(CAST({col} as DATE), 'WW')",
+        'P1M': "TRUNC(CAST({col} as DATE), 'MONTH')",
+        'P0.25Y': "TRUNC(CAST({col} as DATE), 'Q')",
+        'P1Y': "TRUNC(CAST({col} as DATE), 'YEAR')",
     }
 
     @classmethod