How to gets len characters from the middle of a String in Apex?

public with sharing class StringToDateConversion { public static Date ParsedDate (String DateString) { String values; String month; String year; String day; for(integer i = 0; i < DateString.length(); i++) { if(i<4) { year = DateString.substring(0,4); } if(i>=4 && i<=5) { month = DateString.substring(4,6); } if(i>=6 && i<=7) { day = DateString.substring(6,8); } } Date mydate = Date.newInstance(integer.valueOf(year),integer.valueOf(month),integer.valueOf(day)); return mydate; }}