A common requirement in any set of calculations is to create a range of time variants on any measure – Prior Period, Year to Date, Prior Year to Date, Prior Quarter… you think of a time slice and someone will find it useful.
However the downside to this is that in the model you end up maintaining lots of calculations that are all largely doing the same thing. Any good coder likes to parameterise and make code reusable. So how could we do this in Tabular? There is a way that is a very specific variant of the idea of Parameter Tables
Disconnect your Dimensions!
Step one is to unhook your Date Dimension from your fact table. This may seem counter-intuitive, but what it frees you to do is to use the Date dimension as a source of reference data that doesn’t filter your data when you select a date – this simplifies the subsequent calculations significantly. You also need to add to the date dimension all the dates you will need to perform your calculations – Year starts, Prior Year starts, Period starts etc. – this isn’t compulsory but you’ll be grateful later on when you need these dates and don’t have to calculate them on the fly, trust me. Your Date table (I’m going to cease calling it a Dimension, it isn’t any more) will end up looking something like this:
In practice you would hide all the columns apart from the Date as this is the only one that actually gets used by users.
Time for the Variants
Next, we need to create a simple filter table to apply the Time Variant calculations. All it needs is a numeric identifier per variant and a variant name, like so:
This – quite clearly – isn’t the clever bit. The thing to observe with all of these variants is that they create a date range. So what we need to do is calculate the applicable Start and End dates of that range. This is the bit where we are grateful we pre-calculated all those in our Date table. We add two Measures to the table, StartDate and EndDate, which detect which Time Variant is being calculated and then work out the appropriate date, based on the currently selected date. The DAX for StartDate looks like this:
We use a SWITCH statement against the VariantID to detect which Variant we are trying to get the date range start for, then pick the right date from the Date Table. Pre-calculating these in the Date table keeps this part simple.
Add it all up
The final part is to pull these dates into the measure:
This works by using the DATEBETWEEN function to apply a custom date range filter to the Transactions table – which we create dynamically through our StartDate and EndDate calculations.
Our end result:
We can see above that we can for a single selected date, generate a range of Start and End dates and apply those to our single summarising function to create multiple Time Variations.
The sample workbook is here: DAX Time Variants