using System;

public class Program

{

    public static void Main()

    {

        var timestampsy = 1666095215;

        System.DateTime dtDateTime = new DateTime(1970,1,1,0,0,0,0,System.DateTimeKind.Utc);

        dtDateTime = dtDateTime.AddSeconds(timestampsy).ToLocalTime();

        Console.WriteLine("Timestamp from cosmos item is: " + dtDateTime);

        var timeStamp = GetTimestamp(DateTime.Now.AddDays(-15));

        var isObjectWithinDays = dtDateTime > DateTime.Now.AddDays(-15) ? true : false;

        Console.WriteLine("Object is within 15 days of now: " + isObjectWithinDays);

    }

    public static String GetTimestamp(DateTime value)

    {

        return value.ToString("yyyyMMddHHmmssffff");

    }

}