Cron to Quartz parser

Parser.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace WhoGroup.Schedulers
{
    internal static class SchedulerHelper
    {
        public static string GernerateQUARTZExpression(string cronExpression)
        {
            var splitExpression = cronExpression.Split(' ').ToArray();
            var quartzExpression = new List<String>();
            quartzExpression.Add("0");
            var toggleQuartzCompat = false;
            for(int index = 0; index < splitExpression.Length; index++)
            {
                var item = splitExpression[index];
                    // index 0 = minutes
                    // index 1 = hours
                    // these cron definitions should be compatible with quartz so we push them as is
                    if (index == 0 || index == 1) {
                        quartzExpression.Add(item);
                    }
                    
                    // index 2 = DOM = Day of Month
                    if (index == 2) {
                        if (item != "?") {
	                        toggleQuartzCompat = true;
                        }

                        if (item == "*") {
	                        toggleQuartzCompat = false;
	                        item = "?";
                        }
			
                        quartzExpression.Add(item);
                    }
                    // index 3 = Month
                    if (index == 3) {
                        quartzExpression.Add(item);
                    }

                    // index 4 = DOW = Day of Week
	                if (index == 4) {

		                // day of week needs another adjustments - it is specified as 1-7 in quartz but 0-6 in crontab
		                var itemAbbreviated = AdvanceNumber(item);

		                if (toggleQuartzCompat == true) {
			                quartzExpression.Add("?");
		                } else {
			                quartzExpression.Add(itemAbbreviated);
		                }
	                }

            }
            quartzExpression.Add("*");
            return String.Join(" ", quartzExpression.ToArray());
        }

        private static string AdvanceNumber(string str) 
        {
	        var quartzCompatibleStr = ""; 
            int num;
	        Array.ForEach(str.Split('\0'), chr =>{
                if (int.TryParse(chr, out num))
                {
			        quartzCompatibleStr += num+1;
		        } else {
			        quartzCompatibleStr += chr;
		        }
	        });
	        return quartzCompatibleStr;
        }
    }
}