ASP.NET / C#: Using GetTimeSpan() Function

Error: 'System.DateTime' does not contain a definition for 'GetTimeSpan' and no extension method 'GetTimeSpan' accepting a first argument of type 'System.DateTime' could be found (are you missing a using directive or an assembly reference?)

using System;
using System.Linq;
using cAlgo.API;
using cAlgo.API.Internals;

namespace cAlgo
{
    [Robot(TimeZone = TimeZones.UTC, AccessRights = AccessRights.None)]
    public class RoboRobot : Robot
    {
        [Parameter(DefaultValue = 0.0)]
        public double Parameter { get; set; }


        protected override void OnTick()
        {

            Print(DateTime.Now.GetTimeSpan());
        }


        public static string GetTimeSpan(DateTime postDate)
        {
            string stringy = string.Empty;
            TimeSpan diff = DateTime.Now.Subtract(postDate);
            double days = diff.Days;
            double hours = diff.Hours + days * 24;
            double minutes = diff.Minutes + hours * 60;
            if (minutes <= 1)
            {
                return "Just Now";
            }
            double years = Math.Floor(diff.TotalDays / 365);
            if (years >= 1)
            {
                return string.Format("{0} year{1} ago", years, years >= 2 ? "s" : null);
            }
            double weeks = Math.Floor(diff.TotalDays / 7);
            if (weeks >= 1)
            {
                double partOfWeek = days - weeks * 7;
                if (partOfWeek > 0)
                {
                    stringy = string.Format(", {0} day{1}", partOfWeek, partOfWeek > 1 ? "s" : null);
                }
                return string.Format("{0} week{1}{2} ago", weeks, weeks >= 2 ? "s" : null, stringy);
            }
            if (days >= 1)
            {
                double partOfDay = hours - days * 24;
                if (partOfDay > 0)
                {
                    stringy = string.Format(", {0} hour{1}", partOfDay, partOfDay > 1 ? "s" : null);
                }
                return string.Format("{0} day{1}{2} ago", days, days >= 2 ? "s" : null, stringy);
            }
            if (hours >= 1)
            {
                double partOfHour = minutes - hours * 60;
                if (partOfHour > 0)
                {
                    stringy = string.Format(", {0} minute{1}", partOfHour, partOfHour > 1 ? "s" : null);
                }
                return string.Format("{0} hour{1}{2} ago", hours, hours >= 2 ? "s" : null, stringy);
            }
            return minutes.ToString("{0} minutes ago");
        }

    }
}

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