YearToDate (YTD) Previous Year (also QTD, MTD, WTD)

This is simple is you combine ParallelPeriod with the YTD functionality:

WITH
MEMBER [Measures].[Sales Amount YearToDate]
AS
    SUM(YTD( [Date].[Calendar].CurrentMember),[Measures].[Sales Amount])

MEMBER [Measures].[Sales Amount Previous Year]
AS
    SUM(ParallelPeriod( [Date].[Calendar].[Calendar Year], 1,[Date].[Calendar].CurrentMember),[Measures].[Sales Amount])

MEMBER [Measures].[Sales Amount YearToDate Previous Year]
AS
    SUM(YTD( ParallelPeriod( [Date].[Calendar].[Calendar Year], 1,[Date].[Calendar].CurrentMember)),[Measures].[Sales Amount])

PeriodsToDate, YearToDate (YTD), QuarterToDate (QTD), MonthToDate (MTD), WeekToDate (WTD)

PeriodsToDate, YearToDate (YTD), QuarterToDate (QTD), MonthToDate (MTD), WeekToDate (WTD) are all MDX functions which aggregate date from de first date of a chosen period up and an including a given end date.