If the two dates you need to compare are on the same row of data your best bet is to add a calculated field at load time, this can then be used in your expression.
The expression in the load will be something like:
There are two advantages to the approach of working this out at load time. First the field can then be used as a selection and a dimension. Secondly there is a much smaller performance hit at display time.
If the two dates are not on the same row of data then you are probably needing to get into nested aggregations - which is a bit more involved. Post back if that is the case.
