DateTimeOffsetExtensions.cs

using System;

namespace InTheHand
{
    /// <summary>
    /// Helper class for DateTimeOffset.
    /// </summary>
    public static class DateTimeOffsetHelper
    {
        private static DateTimeOffset dt = new DateTimeOffset(1970, 1, 1, 0, 0, 0, TimeSpan.Zero);

        /// <summary>
        /// Converts a Unix time expressed as the number of seconds that have elapsed since 1970-01-01T00:00:00Z to a DateTimeOffset value. 
        /// </summary>
        /// <param name="seconds">A Unix time, expressed as the number of seconds that have elapsed since 1970-01-01T00:00:00Z (January 1, 1970, at 12:00 AM UTC).
        /// For Unix times before this date, its value is negative.</param>
        /// <returns>A date and time value that represents the same moment in time as the Unix time. </returns>
        public static DateTimeOffset FromUnixTimeSeconds(long seconds)
        {
            return dt.AddSeconds(seconds);
        }

        /// <summary>
        /// Returns the number of seconds that have elapsed since 1970-01-01T00:00:00Z. 
        /// </summary>
        /// <param name="date">The DateTimeOffset value.</param>
        /// <returns>The number of seconds that have elapsed since 1970-01-01T00:00:00Z. </returns>
        /// <remarks>Unix time represents the number of seconds that have elapsed since 1970-01-01T00:00:00Z (January 1, 1970, at 12:00 AM UTC).
        /// It does not take leap seconds into account.
        /// <para>This method first converts the current instance to UTC before returning its Unix time.
        /// For date and time values before 1970-01-01T00:00:00Z, this method returns a negative value.</para></remarks>
        public static long ToUnixTimeSeconds(this DateTimeOffset date)
        {
            return Convert.ToInt64(date.Subtract(dt).TotalSeconds);
        }
    }
}