set analysis

[chapil]

Hy guys, 

I want to do a specific set analysis : 

Want to sum every lines (ref lettrage) for which DIFF> 60 jours, DIFF is DCC-GL (dates) in days. But DIFF is not a measure or dimension yet (and I can't create master items)

I ve tested like this but I m sure there is a better way to do it : 

Sum({$ < DIFF= {"=Count({$<[DCC]= {'>$(=date(addmonths(today(),-1), 'DD/MM/YYYY'))'} * {'<$(=date(today(), 'DD/MM/YYYY'))'} >} [DCC]- [GL] ) > 60"}> } [REF LETTRAGE])

Thanks for the help

PS: don't want to use if statement