Is a year-to-date dimension the same as a time dimension? If I have a table such as...:

YYMM | Sales
------------
1201 | 10
1202 | 20
1203 | 30
1204 | 40


... then I might describe this as having a time dimension. If I wanted to calculate a year-to-date value of Sales, would "year to date" be a different dimension to time, or is it actually the same and the additional measure implies the different meaning? Such as:

YYMM | Sales | SalesYTD
-----------------------
1201 | 10    | 10
1202 | 20    | 30
1203 | 30    | 60    
1204 | 40    | 100
Year to date sales is a different measure.  Depending on your platform (OLAP systems, for example) it may be purely calculated on the fly.  If you have an appropriate unit of analysis to do so, store it, otherwise calculate it on the fly.