RFC 3339 Easy.

rfc3339

void Main()
{
    string[] formats = 
    {    
        "yyyy-MM-dd'T'HH:mm:ss.fffffffK",
        "yyyy-MM-dd'T'HH:mm:ss.ffffffK",
        "yyyy-MM-dd'T'HH:mm:ss.fffffK",
        "yyyy-MM-dd'T'HH:mm:ss.ffffK",
        "yyyy-MM-dd'T'HH:mm:ss.fffK",
        "yyyy-MM-dd'T'HH:mm:ss.ffK",
        "yyyy-MM-dd'T'HH:mm:ss.fK",
        "yyyy-MM-dd'T'HH:mm:ssK",
        "yyyy-MM-dd'T'HH:mmK",
        "yyyy-MM-dd'T'HH:mm:ss.fffffffK",
        "yyyy-MM-dd'T'HH:mm:ss.ffffffK",
        "yyyy-MM-dd'T'HH:mm:ss.fffffK",
        "yyyy-MM-dd'T'HH:mm:ss.ffffK",
        "yyyy-MM-dd'T'HH:mm:ss.fffK",
        "yyyy-MM-dd'T'HH:mm:ss.ffK",
        "yyyy-MM-dd'T'HH:mm:ss.fK",
        "yyyy-MM-dd'T'HH:mm:ssK",
        "yyyy-MM-dd'T'HH:mmK"
    };

    // Usage
    foreach (var rfc3339String in RFC3339Generator.GenerateDateTimeSamples())
    {
        if (DateTimeOffset.TryParseExact(rfc3339String, formats, CultureInfo.InvariantCulture, DateTimeStyles.None, out var parsedDate))
        {    
             $"Parsed DateTimeOffset: {string.Format("{0:yyyy-MM-dd'T'HH:mm:ss.fffffffK}", parsedDate)}".Dump(rfc3339String);
        }
        else
        {
            Console.WriteLine("Failed to parse RFC 3339 date string.");
        }
    }
}

public class RFC3339Generator
{
    public static IEnumerable<string> GenerateDateTimeSamples()
    {
        // Base date and time with specific milliseconds
        var baseDateTime = new DateTime(2023, 5, 6, 14, 35, 48, 123, DateTimeKind.Utc);  // 123 milliseconds

        // Time zones to consider: UTC and various offsets
        var timeZones = new List<string> { "Z", "+00:00", "-08:00", "+05:30", "+12:45", "-11:30" };

        // Generate date-times with different levels of precision
        foreach (var timeZone in timeZones)
        {
            // Full date and time with seconds
            yield return baseDateTime.ToString($"yyyy-MM-dd'T'HH:mm:ss{timeZone}");

            // Full date and time with milliseconds and further fractional seconds
            yield return baseDateTime.ToString($"yyyy-MM-dd'T'HH:mm:ss.fff{timeZone}");  // Milliseconds
            yield return baseDateTime.ToString($"yyyy-MM-dd'T'HH:mm:ss.ffff{timeZone}"); // 10 microseconds
            yield return baseDateTime.ToString($"yyyy-MM-dd'T'HH:mm:ss.fffff{timeZone}"); // 100 nanoseconds
            yield return baseDateTime.ToString($"yyyy-MM-dd'T'HH:mm:ss.ffffff{timeZone}"); // 1 microsecond
            yield return baseDateTime.ToString($"yyyy-MM-dd'T'HH:mm:ss.fffffff{timeZone}"); // 0.1 microsecond

            // Generating with hour and minute precision
            yield return baseDateTime.ToString($"yyyy-MM-dd'T'HH:mm{timeZone}");
        }
    }
}