Microsoft To Do Graph API: PATCHing recurrence returns “Invalid JSON, Error converting value … to type 'Microsoft.OData.Edm.Date'”

Hi team — I’m hitting a repeatable issue when PATCHing the recurrence of a To Do task via Microsoft Graph.

Endpoint: PATCH https://graph.microsoft.com/v1.0/me/todo/lists/{list-id}/tasks/{task-id}

What I'm sending (sanitized):
{

"dueDateTime": { "dateTime":"2026-01-15T00:00:00", "timeZone":"UTC" }, "recurrence": {
"pattern": { "type":"weekly", "interval":1, "daysOfWeek":["thursday"] },

"range":   { "type":"endDate", "startDate":"2026-01-15", "endDate":"2026-07-09" }  

}
}

Observed result (HTTP 400):

Invalid JSON, Error converting value "2026-01-15" to type 'Microsoft.OData.Edm.Date'. Path 'recurrence.range.startDate'

(I sometimes get the same message for recurrence.range.endDate.)

Context:

• Language/SDK: .NET using the Kiota-based Microsoft Graph SDK (Microsoft.Graph v5 family).
• Other PATCHes (title, note/body, categories, dueDateTime, status) succeed; the failure happens only when a recurrence payload is included.
• I’ve tried:
  1. Sending recurrence via typed PatternedRecurrence (SDK models).
  2. Sending raw JSON (as above).
  3. Adding firstDayOfWeek and recurrenceTimeZone.
  4. Clearing first, then setting:
     {"recurrence": null}
     followed by the JSON above. The second call still 400s with the same Edm.Date conversion error.
• Dates are yyyy-MM-dd strings (which should map to Edm.Date). Time zone is set on dueDateTime or range.recurrenceTimeZone depending on the attempt.
• I also verified the outgoing body by reading the serialized request stream before sending. Representative logs:
  PATCH payload (sanitized / will-send): {"dueDateTime":{"dateTime":"2026-01-15T00:00:00","timeZone":"UTC"}, "recurrence":{"pattern":{"type":"weekly","interval":1,"daysOfWeek":["thursday"]},

             "range":{"type":"endDate","startDate":"2026-01-15","endDate":"2026-07-09"}}}
  

  HTTP 400: Invalid JSON, Error converting value "2026-01-15" to type 'Microsoft.OData.Edm.Date'. Path 'recurrence.range.startDate'
  Expected The task’s recurrence updates to a weekly series on the specified day. This matches the To Do UI semantics. Actual Server returns 400 “Invalid JSON … Edm.Date” on startDate (or endDate) even though they are ISO yyyy-MM-dd strings. Questions
  1. Is there a known issue with todoTask.recurrence Edm.Date handling?
  2. Is there a documented alternate shape for range.startDate/endDate that To Do expects?
  3. Any request headers or shape differences required when setting recurrence (e.g., omit dueDateTime, include specific recurrenceTimeZone, etc.)?
  I can provide request/response headers (including request-id/client-request-id) if helpful. Thanks!