More C# extensions for the DateTime class.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace CoreSystem.ValueTypeExtension
{
    /// <summary>
    /// Helper functions for DateTime
    /// </summary>
    public static class DataTimeExtension
    {
        private static readonly DateTime UnixRefereceDataTime = (new DateTime(1970, 1, 1, 0, 0, 0, 0)).ToUniversalTime();

        /// <summary>
        /// Time passed since 1st, January 1970
        /// </summary>
        /// <param name="dateTime">Value from to which calculate unix reference</param>
        /// <returns>Total seconds password since 1st, January 1970</returns>
        public static long GetUnixTime(this DateTime dateTime)
        {
            return (long)(dateTime.ToUniversalTime() - UnixRefereceDataTime).TotalSeconds;
        }

        /// <summary>
        /// Time passed since specified value in user friendly string e.g '3 mins ago'
        /// </summary>
        /// <param name="dateTime">Value to convert in user friendly string</param>
        /// <returns>User friendly datetime string e.g '3 mins ago'</returns>
        public static string When(this DateTime dateTime)
        {
            return dateTime.When(DateTime.Now);
        }

        /// <summary>
        /// Time passed since specified value in user friendly string e.g '3 days ago'
        /// </summary>
        /// <param name="dateTime">Value to convert in user friendly string</param>
        /// <param name="currentTime">Value to take reference as current time when converting to user friendly string</param>
        /// <returns>User friendly datetime string e.g '3 days ago'</returns>
        public static string When(this DateTime dateTime, DateTime currentTime)
        {
            var timespan = currentTime - dateTime;

            if (timespan.Days > 365)
                return string.Format("{0} year{1} ago", timespan.Days / 365, (timespan.Days / 365) > 1 ? "s" : "");

            if (timespan.Days > 30)
                return string.Format("{0} month{1} ago", timespan.Days / 30, (timespan.Days / 30) > 1 ? "s" : "");

            if (timespan.Days > 0)
                return string.Format("{0} day{1} ago", timespan.Days, timespan.Days > 1 ? "s" : "");

            if (timespan.Hours > 0)
                return string.Format("{0} hour{1} ago", timespan.Hours, timespan.Hours > 1 ? "s" : "");

            if (timespan.Minutes > 0)
                return string.Format("{0} minute{1} ago", timespan.Minutes, timespan.Minutes > 1 ? "s" : "");

            return "Just now";
        }
    }
}