Assuming you are talking about a Date dimension. Within BIDS there are options when creating an SSAS project to have the tool build a Date (Time) dimension for you. Here is an article that shows one of the options http://www.ssw.com.au/SSW/Standards/Rules/CreatingATimeDimensionIn10EasySteps.aspx.

You need to define a dimension in your SSAS database as type Time and associate the attribute hierarchies to their types in order to utilize Time based function like YTD, MTD, ParallelPeriod, PeriodsToDate, and the built-in Time Intelligence functionality.