Last 'x' weeks with custom week format

Hi,

I need a boolean calculatation field to filter out the last 6 weeks of my date in oder to make a yoy comparison. However, I have a bespoke week format based on the week starting on Monday (ie: Weeknum(Date, 21)). I am struggling to filter last 'x' weeks as the weeks run from 1 to 52. The DATEDIFF function based on the standard 'WEEK' interval does not do the trick due to the different week format that I use.

Thank you