Project Online: cannot group by "week of year"

[Ignanikola]

Does anybody know how to Group by: week of year in Project Online?

I group by date and then choose Units/date format "January 2010" to group by month. However in date format drop down list there is no option for "week of year".

In Desktop App this is achived in Group Definition/ Define Intervals.

Thank you.

[#2]

@Ignanikola I shall manipulate the date by using REST/CSOM then convert the date into WEEKNUM then post back to Project Online to another custom field (for example "Week Number" as integer).