def convert_time_units(unit, frequency):
    """
    Converts default time units from file into time units using the MIP frequency.
    As en example, for a 3-hourly file, the time units "days since YYYY-MM-DD" becomes "hours since YYYY-MM-DD".

    :param str tunits: The NetCDF time units string from file
    :param str frequency: The time frequency
    :returns: The converted time units string
    :rtype: *str*

    """
    units = {'subhr': 'minutes', '3hr': 'hours', '6hr': 'hours', 'day':
        'days', 'mon': 'months', 'yr': 'years'}
    return unit.replace('days', units[frequency])
