This extension class helps to easily convert to/from unix timestamps.

DateTimeExtension.cs

using System;

/// <summary>
/// This extension class helps to easily convert to/from unix timestamps.
/// </summary>
public static class DateTimeExtension
{
	private static readonly DateTime Epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);

	/// <summary>
	/// Convert the passed datetime (in utc) to a unixtimestamp.
	/// </summary>
	/// <param name="utc">A <see cref="DateTime"/> value where Kind is UTC.</param>
	/// <returns>Seconds since unix timestamp epoch.</returns>
	/// <exception cref="ArgumentException">Thrown when 'utc' argument is not of kind utc.</exception>
	public static long ToEpoch(this DateTime utc)
	{
		if (utc.Kind != DateTimeKind.Utc) throw new ArgumentException("The passed DateTime kind is NOT utc.", "utc");
		return (long)((utc - Epoch).TotalSeconds);
	}

	/// <summary>
	/// Convert the passed unix timestamp to a UTC <see cref="DateTime"/> value.
	/// </summary>
	/// <param name="secondsSindsEpoch">Total seconds passed since unix timestamp epoch.</param>
	/// <returns>A <see cref="DateTime"/> where its Kind is UTC. </returns>
	public static DateTime FromEpoch(this long secondsSindsEpoch)
	{
		return Epoch.AddSeconds(secondsSindsEpoch);
	}

	/// <summary>
	/// Rounds a datetime based the interval specified.
	/// </summary>
	public static DateTime Round(this DateTime dt, TimeSpan d)
	{
		return new DateTime(((dt.Ticks + d.Ticks - 1) / d.Ticks) * d.Ticks);
	}
}