Client Status Measure Based on Selected Date

Hi InsightSeeker 
Try below measure

Status = 

var max_month=MAX('Calendar Ultimate'[Date])
//trading dates for last 13 months
var existing_client_dates=DATESINPERIOD('Calendar Ultimate'[Date],max_month,-13,MONTH)
//trading dates for last 12 months
var nbto_dates=DATESINPERIOD('Calendar Ultimate'[Date],max_month,-12,MONTH)

//trading dates for last 6 month
var lost_client_dates=DATESINPERIOD('Calendar Ultimate'[Date],TODAY(),-6,MONTH)

var res_existing_client=CALCULATE(COUNT(data[Order_Number]),existing_client_dates)
var res_newclient=CALCULATE(COUNT(data[Order_Number]),'Calendar Ultimate'[Date]=max_month)
var res_nbto=CALCULATE(COUNT(data[Order_Number]),nbto_dates)
var res_lost_client=CALCULATE(COUNT(data[Order_Number]),lost_client_dates)

RETURN 
SWITCH(
    TRUE(),
    NOT ISBLANK(res_existing_client),"Existing CLient", //more than 12 months
    NOT ISBLANK(res_nbto),"NBTO",   //less than equal to 12 months
    NOT ISBLANK(res_newclient),"NBTO", //zero month
    ISBLANK(res_lost_client) || res_lost_client=0,"Lost CLient" //more than 6 month

)

please give kudos or mark it as resolved once confirmed