diff --git a/agate/aggregations/mean.py b/agate/aggregations/mean.py
index 689b674f..ac0de983 100644
--- a/agate/aggregations/mean.py
+++ b/agate/aggregations/mean.py
@@ -3,7 +3,7 @@
 from agate.aggregations.base import Aggregation
 from agate.aggregations.has_nulls import HasNulls
 from agate.aggregations.sum import Sum
-from agate.data_types import Number
+from agate.data_types import Number, TimeDelta
 from agate.exceptions import DataTypeError
 from agate.warns import warn_null_calculation
 
@@ -20,13 +20,16 @@ def __init__(self, column_name):
         self._sum = Sum(column_name)
 
     def get_aggregate_data_type(self, table):
-        return Number()
+        column = table.columns[self._column_name]
+
+        if isinstance(column.data_type, (Number, TimeDelta)):
+            return column.data_type
 
     def validate(self, table):
         column = table.columns[self._column_name]
 
-        if not isinstance(column.data_type, Number):
-            raise DataTypeError('Mean can only be applied to columns containing Number data.')
+        if not isinstance(column.data_type, (Number, TimeDelta)):
+            raise DataTypeError('Sum can only be applied to columns containing Number or TimeDelta data.')
 
         has_nulls = HasNulls(self._column_name).run(table)