DAX power query

Count = 
// Date must be a real date, not a string.
var MinDate = MIN( T[Date] )
var CodeOfInterest = "SN23"
var CodesFromBefore = {"SN21", "MR28"}
var CasesOfInterest =
    FILTER(
        GROUPBY(
            CALCULATETABLE(
                // This SUMMARIZE finds the pairs of Case ID
                // and Procedure Code in T where dates are
                // before the min date visible and codes
                // are filtered to be only the ones we're
                // interested in.
                SUMMARIZE(
                    T,
                    T[Case ID],
                    T[Procedure Code]
                ),
                T[Date] < MinDate,
                T[Procedure Code] in CodesFromBefore,
                // This CALCULATETABLE returns only
                // the Case ID's we're interested in
                // from the current context.
                CALCULATETABLE(
                    DISTINCT( T[Case ID] ),
                    KEEPFILTERS( 
                        T[Procedure Code] = CodeOfInterest 
                    )
                )
            ),
            // The GROUPBY takes the table from above and
            // checks for each ID how many codes it
            // has. If the number is 2, then we've got
            // the CaseID we've been after. If it's one
            // then one code is missing.
            T[Case ID],
            "@CodeCount", SUMX( CURRENTGROUP(), 1 )
        ),
        // So now we are filtering out the case id's
        // that have just one code.
        [@CodeCount] = 2
    )
var Result = COUNTROWS( CasesOfInterest )
RETURN
    Result