Find Earliest Date from Several Columns

Figured it out!

For all those curious, first I determine the earliest date using: 

Date Entered Funnel = 
VAR AllDates = {
    ([Entered Stage 1]),
    ([Entered Stage 2]),
    ([Entered Stage 3]),
    ([Entered Stage 4]),
    ([Entered Stage 5]),
    ([Entered Stage 6]) }
    VAR MinDate = 
        MINX ( FILTER( AllDates, [Value] <> 0), [Value] )
    RETURN
        MINX( FILTER( AllDates, [Value] = MinDate ), [Value] )

Then I calculated the date between the date reported above, and today's date (unless the Job had completed, which I checked first). 

VelocityTotal All = 
//This function calculates the total velocity for each opportunity.
//It checks if the opportunity has completed first. If so, it reports the time between entering the funnel and completion date.
//If not, it takes the differece between entering the funnel and the current date, and reports it.  
IF(NOT(ISBLANK([Completed Date])), DATEDIFF([Date Entered Funnel], [Completed Date], DAY), 
    DATEDIFF([Date Entered Funnel], TODAY(), DAY))