Compare two date colums

If you want to do this in Power Query, there are several ways. If you want to know which is larger:

if [Date1] > [Date2] then "Date 1" else "Date 2"

If you just want to return the largest date, then

List.Max({[Date1], [Date2]})

The {} brackets put the Date1 and Date2 field in a list and returns the largest. 

In DAX, it is similar:

IF(
   Table[Date1] > Table[Date2],
   "Date 1",
   "Date 2"
)

Or just returning the largest:

MAX(Table[Date1],Table[Date2])

You could substitute measures for those table[date] fields depending on what your report is doing.