Does your expression currently work for one date value?

The MDX syntax for a date range is this: <date1> : <date2>

You should be able to use this in your CUBEVALUE function, as long as it is passed in the same member expression parameter. I haven't tried this before, but if your function works for one date it should merely be a matter of getting the syntax right. Also use a trace to see what MDX is generated by Excel... that could help you troubleshoot the issue (if any).