Describe how to populate value and valueLabel for anchor timings

When describing Timings, the IG says:

>The timing definitions should be read as "the node is the node".

However, this doesn't work for anchor timings, where \ is "Fixed Reference". Both the `value` and `valueLabel` attributes of a Timing must be populated, but:
- The IG does not mention how to populate them for anchor timings or what the "value of a fixed reference timing" would represent.
- There are no CORE rules that check the values of `value` or `valueLabel` for fixed reference timings.

The following `value` / `valueLabel` combinations are used for fixed reference timings in the all of the available example data files:
- P1D / 1 Day
- PT0M / 0 mins

but it's not clear how these should be interpreted.