Attached is an example file showing how to create an 'As At' calendar.
The concept of this is that for each date, you wish to report on facts which occur on or before that date, or perhaps you'd like to see your data as it was in a prior period. So, a trivial example of this might be that you have the following dates in your calendar
Your As At Calendar would be:
As At Date Date
It's then a simple matter to add in flags such as Current Month, Current Quarter, Current Year etc... and allow your users to compare their data 'As At' today, against prior periods.
Note that if you use your As At date as a dimension, your calculations will by default become cumulative. The users date selections and sort order changes will no longer affect accumulation. If you wish to avoud this, you will need to add a current period flag and use this in your expression.