MS Graph create event 400 bad request - microsoft-graph-api

Question

I have code that get's the default calendar and stores the id in a variable called calendar id. I then I then get a list of availabilities (which are events in our system) the end time and the start time are recorded in utc and I want to put those events into an office 365 calendar.
This is my code which is generating and 400 Bad request. Does anyone have any idea what I am doing wrong?
using (HttpClient httpClient = new HttpClient())
{
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", msBearerToken);
var callJson = new
{
Subject = "Booking",
Body = new ItemBody
{
ContentType = BodyType.Html,
Content = "Available"
},
Start = new DateTimeTimeZone
{
DateTime = availabilitySummery.StartDateUTC.ToString(),
TimeZone = "Europe/London"
},
End = new DateTimeTimeZone
{
DateTime = availabilitySummery.EndDateUTC.ToString(),
TimeZone = "Europe/London"
}
};
string path = $"/me/calendars/{calendarId}/events";
// string path = "/me/calendars/events";
path = "https://graph.microsoft.com/v1.0" + path;
HttpResponseMessage responseMessage = await httpClient.PostAsJsonAsync(path, callJson);}

Answer 1

Date should be ISO8601 formatted.
Replace .ToString() by .ToString("o") when you want to convert dates to string.
Ref: The round-trip ("O", "o") format specifier
And createdDateTime property of event object

Answer 2

Update: This worked:
using (HttpClient httpClient = new HttpClient())
{
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", msBearerToken);
var callJson = new
{
Subject = "Booking",
Body = new
{
ContentType = BodyType.Html.ToString(),
Content = "Available"
},
Start = new
{
DateTime = availabilitySummery.StartDateUTC.ToString("yyyy-MM-ddTHH:mm:ss"),
TimeZone = "GMT Standard Time"
},
End = new
{
DateTime = availabilitySummery.EndDateUTC.ToString("yyyy-MM-ddTHH:mm:ss"),
TimeZone = "GMT Standard Time"
}
};
string path = $"/me/calendars/{calendarId}/events";
string jsoncall = JsonConvert.SerializeObject(callJson);
path = "https://graph.microsoft.com/v1.0" + path;
HttpResponseMessage responseMessage = await httpClient.PostAsJsonAsync(path, callJson);
string content = await responseMessage.Content.ReadAsStringAsync();
if (!responseMessage.IsSuccessStatusCode)
{
return false;
}
}