Distinct count on dynamic MIN(datekey)

// Let's say that the table you're showing
// is the fact table, FT, and it's connected
// on the obivous fields to dimensions:
// Customer, Brand, Channel, Date.
// The other fileds, Metric and Venue,
// should also have their own dimensions
// but I'll use Metric as it is. Please
// note as well, that you should never
// drop a column from a fact table onto
// the canvas if there is a dimension
// attached to it. This will almost always
// mess up calculations. Such columns should
// always be hidden.

// Please come up with a better name
// for the measure. The name of the
// measure should give a hint at what
// it really does. Thanks.

[# Cust With First Visit] =
var __startDate = MIN( 'Date'[Date] )
var __endDate = MAX( 'Date'[Date] )
var __result =
    SUMX(
        Customer,
        var __firstDate =
            CALCULATE(
                MIN( FT[Metric Date] ),
                // If you don't want to intersect
                // the filter on Metric with "visit",
                // just remove the keepfilters wrapping
                // and only leave the condition under it.
                KEEPFILTERS( FT[Metric] = "visit" ),
                ALL( 'Date' )
            )
        return
            (__firstDate >= __startDate)
            *
            (__firstDate <= __endDate)
    )
return
    if( __result, __result )