Comparing two date periods can be difficult, and sometimes it requires SQL. You have to modify the dates so that they join correctly, and then use date and time logic to get the period you want, dynamically.
The Date Compare Transform makes this so much easier.
Simply pick the time period you want as your base, then pick what time period you want to compare it to.
Grow will then dynamically place your two periods, side by side with all the data, and dynamically update the date range to what you want. This makes it easy to calculate key values or chart this month against last month.
How to Use:
Click 'Add Transform' then pick 'Compare Dates'
Pick the date column you want to use to compare.
Select the range of your base period.
Select your comparison period.
Click 'Done', and the transform will start calculating.
When it it is done, your base and comparison periods will be laid out side by side.
Use 'Add Data' to select the two data series and you are done.
For Best Results:
Group before you compare
Grouping is a powerful transform that can radically re-arrange and remove columns. Comparing dates after you already have the data grouped how you would like is recommended.