covert EpochTime to Datetime and covert Datetime to EpochTime

 /// <summary>
/// Converts the given date value to epoch time.
/// </summary>
public static long ToEpochTime(this DateTime dateTime)
{
    var date = dateTime.ToUniversalTime();
    var ticks = date.Ticks - new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc).Ticks;
    var ts = ticks / TimeSpan.TicksPerSecond;
    return ts;
}

/// <summary>
/// Converts the given date value to epoch time.
/// </summary>
public static long ToEpochTime(this DateTimeOffset dateTime)
{
    var date = dateTime.ToUniversalTime();
    var ticks = date.Ticks - new DateTimeOffset(1970, 1, 1, 0, 0, 0, TimeSpan.Zero).Ticks;
    var ts = ticks / TimeSpan.TicksPerSecond;
    return ts;
}

/// <summary>
/// Converts the given epoch time to a <see cref="DateTime"/> with <see cref="DateTimeKind.Utc"/> kind.
/// </summary>
public static DateTime ToDateTimeFromEpoch(this long intDate)
{
    var timeInTicks = intDate * TimeSpan.TicksPerSecond;
    return new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc).AddTicks(timeInTicks);
}

/// <summary>
/// Converts the given epoch time to a UTC <see cref="DateTimeOffset"/>.
/// </summary>
public static DateTimeOffset ToDateTimeOffsetFromEpoch(this long intDate)
{
    var timeInTicks = intDate * TimeSpan.TicksPerSecond;
    return new DateTimeOffset(1970, 1, 1, 0, 0, 0, TimeSpan.Zero).AddTicks(timeInTicks);
}