RecurrenceRule = "FREQ=DAILY;UNTIL=20210722T000000"

RecurrenceRule = "FREQ=DAILY;UNTIL=2021-07-22T00:00:00"

RecurrenceRule = "FREQ=DAILY;UNTIL=2021-07-22T00:00:00.000Z"

private string TransformRecurrenceRule()
        {
            const string untilSeparator = "UNTIL=";
            var ruleParts = RecurrenceRule.Split(untilSeparator, StringSplitOptions.RemoveEmptyEntries);
            if (ruleParts.Length <= 1)
            {
                // There was no Until clause to worry about
                return RecurrenceRule;
            }

            // Save the first part of the rule
            var ruleBeginning = ruleParts[0];

            // Split the date part of the until clause from any following clauses
            var remainingClauses = ruleParts[1].Split(';', 2, StringSplitOptions.RemoveEmptyEntries);

            //Save the date part of the until clause
            var untilDate = remainingClauses[0];

            // Save any following clauses with the `;` replaced
            var ruleEnding = "";
            if (remainingClauses.Length == 2)
            {
                ruleEnding = $";{remainingClauses[1]}";

            }

            // Convert the until date into .net parsable format
            const string format = "yyyyMMddTHHmmss";
            var date = DateTime.ParseExact(untilDate, format, CultureInfo.InvariantCulture);
            var dateStr = date.ToString("yyyy-MM-ddTHH:mm:ss");

            // recombine rule components
            var newRuleParts = new[] {ruleBeginning, untilSeparator, dateStr, ruleEnding};
            var newRule = string.Join("",newRuleParts);

            return newRule;
        }