Validate the date is preceding to the date

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C#

Repeater rptdates = new Repeater();
                rptdates = (Repeater)rpt1.Items[e.Item.ItemIndex].FindControl("rptLevel2");
                string BatchDates="";
                string dates = "";

                  DropDownList ddllist = new DropDownList();
                LinkButton LnkLevel2 = new LinkButton();
                int flag = 0;
                string PKGMinorCode = "";

                for (int i = 0; i < rptdates.Items.Count; i++)
                {
                    ddllist = (DropDownList)rptdates.Items[i].FindControl("ddlDate");
                    BatchDates = BatchDates + ddllist.SelectedValue.ToString().Trim() + "+";
                    dates += ddllist.SelectedItem.Text.ToString() + ",";

                    LnkLevel2 = (LinkButton)rptdates.Items[i].FindControl("lnkLevel2");
                    PKGMinorCode += LnkLevel2.CommandArgument.ToString().Trim() + ",";
                }

                string[] Coursecheck;
                datecheck = dates.Split(',');
                Coursecheck = PKGMinorCode.Split(',');
                int len = datecheck.Length;
                for (int z = 0; z < len-1; z++)
                {
                    for (int j = z + 1; j < len-1 ; j++)
                    {
                   
                        //PSCRB Date must be preceding to  AFF Date
                        if (Coursecheck[z] == "RPSCRB" && Coursecheck[j] == "R-AFF")
                        {
                            if (datecheck[z] != datecheck[j])
                            {
                                ScriptManager.RegisterStartupScript(this, GetType(), "Invalid date", "alert('RPSCRB and R-AFF Dates must not be the same date')", true);
                                return;
                            }
                        }

in run mode i select the AFF Date after than PSCRB Date that time message shows
PSCRB Date must be preceding to AFF Date

From my above code how can i do?


AFF Date in dropdownlist as follows;

16 May 2013

PSCRB Date in Dropdownlist as follows;

19 May 2013

if (datecheck[z] != datecheck[j])

in the above line how can i check PSCRB Date must be preceding to AFF Date


Output i want as follows;

AFF Date in dropdownlist as follows;

15 May 2013

PSCRB Date in Dropdownlist as follows;

14 May 2013


Please help me

Regards,
Narasiman P.