Please help me to Generate number

public class MyPointlessClass
{
    DateTime lastTime = new DateTime(0);
    Int64 myNumber = 0;

    // as a string
    private string UpdateRunCount()
    {
        DateTime now = DateTime.Now;
        if (now.Year != lastTime.Year)
        {
            myNumber = 1;
        }
        string number = string.Format("{0}{1:0000000000}", now.ToString("yyyy"), myNumber);
        lastTime = 
        return number;
    }

    // as a numeric value
    private Int64 UpdateRunCount()
    {
        DateTime now = DateTime.Now;
        if (now.Year != lastTime.Year)
        {
            myNumber = 1;
        }
        Int64 number = (now.Year * 10000000000) + myNumber;
        return number;
    }

}