Dynamic Grouping of crosstab in BIRT

I have a crosstab which have 3 groups. a)Date b)Code c)count of orders(aggregation -sum) .
I have 2 report parameters 'start_date' and 'end_date'.
Based on my input I need to calculate the difference between the dates
example : var diff = start_date - end_date --------- which gives the no of days.
if(diff <15)
Group the "Date group" to display as dates
if(diff >15)
{
Group the "Date group" to display as weeks
{ if(diff > 30 )
Group the "Date group" to display as months
{ if(diff>365)
Group the "Date group" to display as year
}
}

Example : start_date : 2016-10-10 ------------- Group by week
end_date : 2016-11-05
var diff = 25 days

display :
Code1 Code2 Code3
2016-10-10 : 5 6 8
2016-10-17: 4 10 6
2016-10-23: 8 5 9

Example : start_date : 2016-10-10 ------------- Group by month
end_date : 2016-12-10
var diff = 60 days

display :
Code1 Code2 Code3
2016-10-10 : 5 6 8
2016-11-10: 4 10 6
2016-12-10: 8 5 9

Example : start_date : 2016-10-10 ------------- Group by day
end_date : 2016-10-15
var diff = 5 days

display :
Code1 Code2 Code3
2016-10-10 : 5 6 8
2016-10-11: 4 10 6
2016-10-12: 8 5 9
2016-10-13: 6 7 3

In the above fashion I need to generate my output based on the no of days.(grouping them)
For crosstab I came to know we need to include the script in onCreate(), onPrepare() etc.
Can anyone please guide me through this scripting or if i can directly edit in the report for the crosstab paramaerts else any link which I can refer.
Thanks in Advance