The function DateInterval.WeekOfYear computes week 53 for 2018 which does not exist.
my full expression is run on 7 of january 2019
= (CStr(format(dateadd("d",-7,now),"yy"))+"W"+right(CStr(100+DatePart(DateInterval.WeekOfYear, dateadd("d",-7,now), vbMonday, vbFirstFourDays)),2))
Results in 18W53
I know it is useless to ask Microsoft to fix anything but keep guys in mind that timedate calculations from Microsoft are not reliable (just as anything they do) :)
i know there can be bugs but Microsoft is terrible at fixing them. Speak not from my emotion but experience. :)
From Project Techcenter