DateTime serialisation

public static class DateTimes
{
    const string W3CDateTimeOffsetFormatString = "yyyy-MM-ddTHH:mm:sszzz";
    const string W3CDateTimeFormatString = "yyyy-MM-ddTHH:mm:ss";

    public static string ToW3COffset(DateTimeOffset? dto)
    {
        return dto.HasValue ? dto.Value.ToString(W3CDateTimeOffsetFormatString) : null;
    }

    public static string ToW3C(DateTime? dt)
    {
        return dt.HasValue ? dt.Value.ToString(W3CDateTimeFormatString) : null;
    }

    public static DateTimeOffset? FromW3COffSet(string w3CDateTimeOffsetString)
    {
        return !String.IsNullOrEmpty(w3CDateTimeOffsetString)
            ? DateTimeOffset.ParseExact(w3CDateTimeOffsetString, W3CDateTimeOffsetFormatString, null, DateTimeStyles.None)
            : (DateTimeOffset?) null;
    }

    public static DateTime? FromW3C(string w3CDateTimeString)
    {
        return !String.IsNullOrEmpty(w3CDateTimeString)
            ? DateTime.ParseExact(w3CDateTimeString, W3CDateTimeFormatString, null, DateTimeStyles.None) 
            : (DateTime?) null;
    }
}