How to Generate Unique Number of 8 digits?

Question

I am using this code to generate a 8 digit unique number.

byte[] buffer = Guid.NewGuid().ToByteArray();
return BitConverter.ToUInt32(buffer, 8).ToString();
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Answer 1

If you express the no as a combination of day( 2digit), hour(2digit), minute(2digit), second(2digit) and year ( 4 digit) , then it will be 12 digit but always unique no.

 DateTime _now = DateTime.Now;
 string _dd = _now.ToString("dd"); //
 string _mm = _now.ToString("MM");
 string _yy = _now.ToString("yyyy");
 string _hh = _now.Hour.ToString();
 string _min = _now.Minute.ToString();
 string _ss = _now.Second.ToString();

 string _uniqueId= _dd+ _hh+ _mm+_min+_ss + _yy;