Soheil Bakshi gives us two methods to pad those numbers:
There are some cases that we want to add a leading zero to a digit, such as showing 01 instead of 1, 02 instead of 2 and so on. We have two options to do this in Power BI, doing it in Power Query or doing it with DAX.
Click through for the two methods, and for a bonus item which incidentally bolsters the argument that dates should be YYYY-MM-DD.