var smonth = document.getElementById("UID_1618316373773").value; 
var sday = document.getElementById("UID_1618316366288").value; 
var syear = document.getElementById("UID_1618316379935").value; 
var emonth = document.getElementById("UID_1618316377101").value; 
var eday = document.getElementById("UID_1618316369054").value; 
var eyear = document.getElementById("UID_1618316382164").value; 

var date = "$st" + sday + smonth + syear + "_000000$et" + eday + emonth + eyear + "_000000";
var yourfile = "$dtHT$ftTfnyourdata.csv" + date;
var url = "http://192.168.50.123/rcgi.bin/ParamForm?AST_Param=" + yourfile;

  window.open( url , "download"); //downloads the file that was just generated