Some useful extensions of System.DateTime

SystemDateTimeExtensions.cs

namespace System
{
  public static class DateTimeExtensions
  {
    public static string ToIso8601String(this DateTime thisDateTime)
    {
      return thisDateTime.ToString("yyyy-MM-ddTHH:mm:ss.fffK");
    }

    public static string ToIso8601String(this DateTimeOffset thisDateTime)
    {
      return thisDateTime.ToString("yyyy-MM-ddTHH:mm:ss.fffK");
    }

    public static DateTime Replace(this DateTime thisTime, int? year = null, int? month=null, int? day = null, int? hour = null, int? minute = null, int? second=null, int? millisecond=null)
    {
      return new DateTime(
        year.GetValueOrDefault(thisTime.Year),
        month.GetValueOrDefault(thisTime.Month),
        day.GetValueOrDefault(thisTime.Day),
        hour.GetValueOrDefault(thisTime.Hour),
        minute.GetValueOrDefault(thisTime.Minute),
        second.GetValueOrDefault(thisTime.Second),
        millisecond.GetValueOrDefault(thisTime.Millisecond)
      );
    }

    public static DateTimeOffset Replace(this DateTimeOffset thisTime, int? year = null, int? month = null, int? day = null, int? hour = null, int? minute = null, int? second = null, int? millisecond = null, TimeSpan? offset = null)
    {
      return new DateTimeOffset(
        year.GetValueOrDefault(thisTime.Year),
        month.GetValueOrDefault(thisTime.Month),
        day.GetValueOrDefault(thisTime.Day),
        hour.GetValueOrDefault(thisTime.Hour),
        minute.GetValueOrDefault(thisTime.Minute),
        second.GetValueOrDefault(thisTime.Second),
        millisecond.GetValueOrDefault(thisTime.Millisecond),
        offset.GetValueOrDefault(thisTime.Offset)
      );
    }
  } 
}