How retrieve number of members in a sublevel of hierarchy

Hi all,
i have a question about how to calculate the number of items (not null) that are contained in a hierarchy level:

for example i have the hierarchy TIME:
Time:
year
quarter
month
day

I use it in a crosstab:
in the rows i put some measure and in the column i put Quarter level and , nested, month level (i want to visualize both quarter and monthly detail of the measure in the row of the crosstab)

The problem is how to retrieve the number of month for each quarter that are not null:
for example , my report time period is only 1 year, from January to December, but in each quarter some months are empty of value (es only January is present in the 1 quarter, only April is present in the 2 quarter and so on...) (see later)

Q1:
Jan
Q2:
Apr
Q3:
Aug
Sept
Q4:
Oct
Nov
Dec

In this example, i want to retrieve (using a DataItem) a number that is equal 1 for Q1, 1 for Q2, 2 for Q3 and 3 for Q4

Someone can help me ?
Tanks

Sim