Custom weeknum formula - Beginning 4th Jan :-(

Happy new year all

Hoping someone can help. I use the =WEEKNUM(A1,2) formula in loads of my spreadsheets. For some unknown reason my company have started week 1 this year on 4th Jan which means a lot of the automated reports I run are having to have dates manually updated.

Is there a work around for this or a custom formula I could use?

Thanks
Dan