def get_time_type(var):
    """
    Inspect a variable and return it's time type, which can be either
    None: not a time variable
    'datetime': is a date/time variable
    'delta': is a timedelta variable
    """
    vartype = str(type(var.values.ravel()[0]))
    if 'time' in vartype:
        if 'delta' in vartype:
            return 'delta'
        else:
            return 'datetime'
    else:
        return None