You have to have a date table with weeks which are numbered within each year with the same scheme (1,2,3...,52/53). Then it'll be easy to move forward and backward in such a calendar.
// Say you have such a calendar.
// Say, your base measure is [Base].
// Also, say that the calendar also
// has a column with False for each
// day apart from today which will
// have True assigned. Name the
// hidden column Dates[IsToday].
// You should also have 2 other columns.
// Dates[YearWeekNumber] will be
// the column that numbers the weeks
// within a year, always restarting at
// the beginning of next year. The other
// will be Dates[AbsoluteWeekNumber]
// and will hold numbers from 1 to the
// number of all weeks in the calendar
// consecutively. This is to be able
// to move easily through weeks.
// Bear in mind, please, that no matter
// what units of time you use in your
// model, be it weeks only, months only,
// years only, the Dates table must always
// be a proper date table with all the days
// covering all the years. Some columns in
// such a table can be hidden (since they
// don't apply or are not meant to be used
// by the end user), but they must be present
// for everything to work correctly, especially
// the time-intel functions.
//
// Then:
[Last Week] =
var CurrentWeek_AbsoluteWeekNumber =
calculate(
selectedvalue( Dates[AbsoluteWeekNumber] ),
Dates[IsToday], // this is a logical column
all( Dates )
)
var LastWeekValue =
calculate(
[Base],
Dates[AbsoluteWeekNumber] = CurrentWeek_AbsoluteWeekNumber - 1,
all( Dates )
)
return
LastWeekValue
[Last Week PY] =
var CurrentWeek_AbsoluteWeekNumber =
calculate(
selectedvalue( Dates[AbsoluteWeekNumber] ),
Dates[IsToday], // this is a logical column
all( Dates )
)
var PrevWeek_AbsoluteWeekNumber = CurrentWeek_AbsoluteWeekNumber - 1
var PrevWeek_YearNumber =
calculate(
selectedvalue( Dates[YearNumber] ),
Dates[AbsuluteWeekNumber] = PrevWeek_AbsoluteWeekNumber,
all( Dates )
)
var PrevWeek_YearWeekNumber =
calculate(
selectedvalue( Dates[YearWeekNumber] ),
Dates[AbsoluteWeekNumber] = PrevWeek_AbsoluteWeekNumber,
all( Dates )
)
var PrevWeek_PYValue =
calculate(
[Base],
treatas(
{PrevWeek_YearNumber - 1, PrevWeek_YearWeekNumber},
Dates[YearNumber],
Dates[YearWeekNumber]
),
all( Dates )
)
return
PrevWeek_PYValue
// the other measures can be constructed similarly...
