$beginmember(qname,offset) returns the start of the period at the same level as the time-dimension member given by the fully-qualified name, and offsets number of periods away at the same level.
The following example returns the start of Fiscal/FY 2005/November:
$beginmember(Fiscal/FY 2005/August,3)
For ForecastPro integration, the parameters can be field references; for example:
$beginmember(${week},${offset})