Last Value

Hey, assuming your table looks like this

and you want to create a Calculated Column "PreviousDateTime" (Calculated Column means your are looking for DAX solution)
then this DAX statement finds the previous DATETTIME value in column "aDateTimeColumn":

PreviousDateTime = 
var currentDateTime = 'FindPreviousDateTime'[aDateTimeColumn]
return
CALCULATE(
    MAXX(
        ALL('FindPreviousDateTime'[aDateTimeColumn])
        ,IF('FindPreviousDateTime'[aDateTimeColumn] < currentDateTime, 'FindPreviousDateTime'[aDateTimeColumn], BLANK())
    )
)

Maybe you will think this code is a little to much, but beeing in a ROW Context of a table (we want to create a CALCULATED COLUMN), this helps to avoid the awkward message "... circular dependency ..." :-)

Hope this helps

Tom