Gets the internal rate and reference time of this.
See set_calibration for more information.
internal, external, rate_num, and rate_denom can be left null if the
caller is not interested in the values.
| this |
a Clock |
| external |
a location to store the external time |
| rate_num |
a location to store the rate numerator |
| rate_denom |
a location to store the rate denominator |
| internal |
a location to store the internal time |