C# Extension Methods to handle ISO-8601 with the same exact ISOFormat used in Javascript

ISOFormatExtensions.cs

public static class ISOFormatExtensions
{
    const string ISOFORMAT = "yyyy-MM-dd\\THH:mm:ss.fffK"; //ISO-8601 used by Javascript (ALWAYS UTC)
    public static string toISOString(this DateTime d, bool useLocal = false) {
        if (!useLocal && d.Kind == DateTimeKind.Local) { 
            //If d is LT or you don't want LocalTime -> convert to UTC and always add K format always add 'Z' postfix
            return d.ToUniversalTime().ToString(ISOFORMAT);
        } else { //If d is already UTC K format add 'Z' postfix, if d is LT K format add +/-TIMEOFFSET
            return d.ToString(ISOFORMAT);
        }
    }
    public static DateTime fromISOString(this DateTime d, string s, bool useLocal = false) {
        //Return a new DateTime parsed used ISOFORMAT - YOU MUST PASS A STRING ENDING WITH 'Z' OR +/-TIMEOFFSET
        var l = DateTime.ParseExact(s, ISOFORMAT, System.Globalization.CultureInfo.InvariantCulture);
        return useLocal ? l : l.ToUniversalTime(); //If you don't set useLocal returned date is always Kind=UTC
    }
    public static DateTime fromISOString(this DateTime d, string date, string time, bool useLocal = false) {
        //Return a new DateTime buiding an ISOFROMAT string from date, time params expressed in UTC (by default) or in LT if you set useLocal=true 
        var sb = new System.Text.StringBuilder(30);
        if (!string.IsNullOrEmpty(date)) { sb.Append(date); sb.Replace('-', '/'); }
        if (!string.IsNullOrEmpty(time)) { sb.Append(' '); sb.Append(time); }
        var s = sb.ToString();
        if (!useLocal) { //Always return DateTime Kind=UTC, if you don't pass +/-TIMEOFFSET or 'Z' postfix I'll add it by default (as needed for UTC)
            if (!(s.Contains('Z') || s.Contains('+') || s.Contains('-'))) s += "Z";
            return d = DateTime.Parse(s, System.Globalization.CultureInfo.InvariantCulture, System.Globalization.DateTimeStyles.AdjustToUniversal | System.Globalization.DateTimeStyles.AssumeUniversal);
        } else { //Return DateTime Kind=Local and do necessary conversion to LT if you pass time with +/-TIMEOFFSET or referred as UTC with 'Z' postfix
            return d = DateTime.Parse(s, System.Globalization.CultureInfo.InvariantCulture, System.Globalization.DateTimeStyles.AssumeLocal);
        }
    }
}

//SAMPLE USING Extension Methods IN C#
var u = DateTime.UtcNow;
var n = DateTime.Now;
Console.WriteLine("NOW {0} n.KIND={1} -> {2} in LT {3}",n,n.Kind,n.toISOString(),n.toISOString(true));
Console.WriteLine("NOWUTC {0} u.KIND={1} -> {2} in LT {3}",u,u.Kind,u.toISOString(),u.toISOString(true));
var ok = new DateTime().fromISOString(n.toISOString(),true);
var okZ = new DateTime().fromISOString(u.toISOString());
Console.WriteLine("ASSERT NOW {0} {1}",(ok-n).TotalMilliseconds, ok.Kind);
Console.WriteLine("ASSERT NOWUTC {0} {1}",(okZ-u).TotalMilliseconds, okZ.Kind);
Console.WriteLine("-------------------------------------------------");
var z = new DateTime().fromISOString("2018-04-26","14:46:23.456Z"); //14.46Z
Console.WriteLine("Z UTC {0} KIND={1} ISO={2}",z,z.Kind,z.toISOString());  
var y = new DateTime().fromISOString("2018/04/26","14:46:23.456");  //14.46Z
Console.WriteLine("Y LT+TOFF {0} KIND={1} ISO={2}",y,y.Kind,y.toISOString());
var x = new DateTime().fromISOString("2018/04/26","14:46:23.456+02:00"); //12.46Z
Console.WriteLine("X LT->UTC {0} KIND={1} ISO={2}",x,x.Kind,x.toISOString());
Console.WriteLine("-------------------------------------------------");
var l = new DateTime().fromISOString("2018/04/26","14:46:23.456",true);  //12.46Z <- LT=14.46
Console.WriteLine("L LT {0} KIND={1} ISO={2}",l,l.Kind,l.toISOString());
var t = new DateTime().fromISOString("2018/04/26","14:46:23.456+02:00",true); //12.46Z
Console.WriteLine("T LT+TOFF {0} KIND={1} ISO={2}",t,t.Kind,t.toISOString());
var c = new DateTime().fromISOString("2018-04-26","14:46:23.456Z",true); //14.46Z -> LT=16.46
Console.WriteLine("C UTC->LT {0} KIND={1} ISO={2}",c,c.Kind,c.toISOString());

When you want to pass DateTime to JSON / Javascript simply serialize the value using toISOString()

//USING Date OBJECT IN JAVASCRIPT 
const strUTC = "2018-04-26T14:46:23.456Z"; //String serialized with extension method DateTime.toISOString(); //UTC
const strLocal = "2018-04-26T16:46:23.456+02:00";  //String serialized with DateTime.toISOString(true); //LT+TIMEOFFSET

Date.parse(strUTC); 
//-> 1524753983456
Date.parse(strLocal)
//-> 1524753983456
new Date(strUTC).toISOString()
//-> "2018-04-26T14:46:23.456Z"
new Date(strLocal).toISOString()
//-> "2018-04-26T14:46:23.456Z"

I suggest to rename the methods in camel-casing for respect the Microsoft naming guideline (seeing that the file is c#)
https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/capitalization-conventions

You meant Pascal Casing ?

Note: C# support new DateTime(string), and for a ISO string, a ToString("o") is enough, isn't it?

Please see: https://docs.microsoft.com/en-us/dotnet/standard/base-types/standard-date-and-time-format-strings

Hi @mmalka yes I think that for toISOString can be simplified with the ToString("o") formatting, I don't know when it was added, I'm pretty sure that 2 year ago it wasn't available, so I create this Extension method to simplify my works to correctly format and parse ISO-8601 format in C#

PS: do you know if there is something like the "o" format even to simplify the parse?