ENH: do not convert mixed-integer type indexes to datetimeindex

Author: cpcloud

pandas/tseries/index.py

@@ -910,7 +910,8 @@ def join(self, other, how='left', level=None, return_indexers=False):
         """
         See Index.join
         """
-        if not isinstance(other, DatetimeIndex) and len(other) > 0:
+        if (not isinstance(other, DatetimeIndex) and len(other) > 0 and
+            other.inferred_type != 'mixed-integer'):
             try:
                 other = DatetimeIndex(other)
             except TypeError: