DataFrame.join can not have datetime as column names
Issue:  
    For DataFrameA.join( DataFrameB , on = 'somecol' ), DataFrameB can not have datetime objects as column labels

Example: 

from datetime import datetime
from pandas import DataFrame

str_dates = [ '20120209'         , '20120222'         ]
dt_dates  = [ datetime(2012,2,9) , datetime(2012,2,22)]

A = DataFrame(str_dates     , index = range(2)  , columns = ['aa']   )

B = DataFrame([[1,2],[3,4]] , index = str_dates , columns = str_dates)
C = DataFrame([[1,2],[3,4]] , index = str_dates , columns = dt_dates )

works = A.join( B , on = 'aa' ) # works -- extra column labels are string
fails = A.join( C , on = 'aa' ) # fails -- extra column labels are datetime
