This code is converting meridian time into military time can you find any test cases it is missing [closed]

My code is failing one test case according to my company software I cant find out what test case it is, as those test cases are hidden. The below Code converts meridian time format into military time format. I would like to not use the datetime functions of c# if possible and find out whats missing in this code itself

public static void Main()
{
    string time = Console.ReadLine();
    string timeHour = time.Substring(0,2);
    string timeRest = time.Substring(2,6);
    string meridian = time[8]+""+time[9];
    string milTime = "";

    // System.Console.WriteLine(meridian);
    // System.Console.WriteLine(timeHour);
    // System.Console.WriteLine(timeRest);
    
    int a =0;

    if(meridian == "PM")
    {
        if(timeHour != "12")
        {
            a = Convert.ToInt32(timeHour)+12;
        }
        else
        {
            a = 12;
        }
    }
    else if(meridian == "AM")
    {
        if(timeHour != "12"){
        a = Convert.ToInt32(timeHour);
        }
        else
        {
            a = 0;
        }
    }
    
    // System.Console.WriteLine(a);
    
    if(a !=0 )
    {
        milTime = a + timeRest;            
    }
    else
    {
        milTime = "00" + timeRest;
    }
    System.Console.WriteLine(milTime);
}