I found the solution myself.
I first setup a field that contains the month the task finished in, in respect to the month the project starts.
This requires a workflow rule as the field based on a formula can only use fields of the same object.
as the month are labeled 1 to 12, the workflow rule first as to check whether the duedate is in the same year as the project startdate:
1. Conditional action list
if True =
Year($DueDate)-Year($ParentProject.StartDate) = 0
1.1 Update Field
2. Conditional action list
If True =
Year($DueDate)-Year($ParentProject.StartDate) = 1
2.1 Update Field
Month($DueDate)-Month($ParentProject.StartDate) + 1 +12
This has to be continued for as many years your projects tend to tun. (I've added the +1 to make month 1 = 1 and not 0)
Following this workflow rule, I have created a custom field that calculates the Project quarter the task ends in. this can be done via a custom field that is based on a formula since all required pieces of information are within the same object.
(The Field is a picklist in my case so I can use it in reports for grouping in cluster graphs)
The field has as picklist entries:
Q1, Q2 , Q3, Q4, Q5, Q6, Q7, Q8, Q9, Q10, Q11, Q12
and the formula to calculate the quarter is a conditional statement that groups the month into quarters:
Again this would need to be adapted depending on how long your projects run.
This obviously falls apart if a task starts and ends in different quarters, so tasks have to be split so they start and end in the same quarter. I have created further custom fields that essential do the same as above, but for the tasks start date and at the end a field that compares the project start quarter to the end quarter. If they are in separate quarters the quarter number is highlight to make the user aware that this task requires splitting.
I know this is a "workaround" rather than using the time-phased information, but this way I can use the baseline apps currently available.
Hope this might help someone