uniqueNumberPerDate

    public class UniqueNumber
    {
        private const long MaxNumbersPerDay = 10000000;
        private readonly object _mutex = new object();
        private long _runningNumber = 0;
        private DateTime _lastDate = DateTime.MinValue;

        public long GetNext()
        {
            lock (_mutex)
            {
                var now = DateTime.UtcNow.Date;
                if (now != _lastDate)
                {
                    _runningNumber = 0;
                    _lastDate = now;
                }
                _runningNumber++;
                var datePart = now.Year*10000 + now.Month*100 + now.Day;
                return datePart*MaxNumbersPerDay + _runningNumber;
            }
        }
    }