I am looking to create a variance calculation for change in 'amount due' over months. The date column I use is "Invoice Date" and I was hoping to create a column that shows the variance from jan to feb, feb to march, etc.
You can do this in Magic ETL or in a Beast Mode if you get a feature enabled.
In Magic ETL, you can utilize the Group By tile to sum up totals by month and year. You can then use the Rank & Window tile and the LAG function to put the previous month total next to the next month and then calculate the difference.
In Beast Mode, I would refer you to this posting that does a nice job of how to do it.