def getstdev(data, dt_start=None, dt_end=None):
    """ Returns a dict with the standard deviation of numeric columns over the given time period.

    Args:
        data (dataframe): The panadas dataframe containing at least a debit and a credit column.
        dt_start (str): The start date (specific if given '2012-11-11' or the month '2012-11')
            from were the standard deviation should be calculated.
        dt_end (str): The end date (specific if given '2012-11-11' or the month '2012-11')
            to were the standard deviation should be calculated.

    Returns:
        A pandas dataframe containing a single standard deviation for each colum in the input dataframe.

    """
    stdev = data[dt_start:dt_end].std()
    return stdev[stdev.notnull()]
