flat file datetime issue

I have a flat file with this datetime format 5/24/2017 05:12:44.243 PM. I have been struggling to convert this format to something like this 2017-05-24 17:12:44.243. I tried using this derived column expression (DT_DBTIMESTAMP)(SUBSTRING(EventDate,FINDSTRING(EventDate,"/",2) + 1,4) + "-" + SUBSTRING(EventDate,1,FINDSTRING(EventDate,"/",1) - 1) + "-" + SUBSTRING(EventDate,FINDSTRING(EventDate,"/",1) + 1,FINDSTRING(EventDate,"/",2) - FINDSTRING(EventDate,"/",1) - 1) + " " + SUBSTRING(EventDate,FINDSTRING(EventDate," ",1) + 1,8)) where "EventDate" is the column name but I am getting the result in this format "2017-05-22 05:21:00.000" also this is not converting the 12 hour to 24 hour. I am new to SSIS development and been struggling on this really long. Please help me on this, any help will be really appreciated.