Calculate previous month

JonnyW , use a separate date table, marked as date table

example

last MTD Sales = CALCULATE(SUM(Sales[Sales Amount]),DATESMTD(dateadd('Date'[Date],-1,MONTH)))
last month Sales = CALCULATE(SUM(Sales[Sales Amount]),previousmonth('Date'[Date]))

Why Time Intelligence Fails - Powerbi 5 Savior Steps for TI :https://youtu.be/OBf0rjpp5Hw
https://amitchandak.medium.com/power-bi-5-key-points-to-make-time-intelligence-successful-bd52912a5bd4
To get the best of the time intelligence function. Make sure you have a date calendar and it has been marked as the date in model view. Also, join it with the date column of your fact/s. Refer :radacad sqlbi My Video Series Appreciate your Kudos.

Power BI — Month on Month with or Without Time Intelligence
https://medium.com/@amitchandak.1978/power-bi-mtd-questions-time-intelligence-3-5-64b0b4a4090e
https://www.youtube.com/watch?v=6LUBbvcxtKA