Main note: it doesn't make sense!
You should use date data type, not string data type, because of several reasons. One of them is that, that you won't be able to make calculations. Anytime you decide to make calculations, you'll be in need to encode/decode date.
BUT(!)... if you want to generate user defined date field, check this:
Dim oDic = New Dictionary(Of Integer, String)
oDic.Add(0, "A")
oDic.Add(1, "B")
oDic.Add(2, "C")
oDic.Add(3, "D")
oDic.Add(4, "E")
oDic.Add(5, "F")
oDic.Add(6, "G")
oDic.Add(7, "H")
oDic.Add(8, "I")
oDic.Add(9, "J")
Dim sDate = "2017/10/11"
sDate = sDate.Replace("/", String.Empty)
Dim encodedDate = String.Join(String.Empty, sDate.Select(Function(x) oDic(Int32.Parse(x))))
Console.WriteLine("encoded date: {0}", encodedDate)
Dim cu = New Globalization.CultureInfo("en-US")
Dim decodedDate = DateTime.ParseExact( _
String.Join(String.Empty, encodedDate.Select(Function(x) oDic.Where(Function(r) r.Value=x).Select(Function(k) k.Key).First())), _
"yyyyMMdd", cu)
Console.WriteLine("decoded date: {0}", decodedDate)
Result:
encoded date: CABHBABB
decoded date: 2017-10-11 00:00:00
Now you know how do encode/decode date. All you need to do right now is to write code to save/retrieve data to/from database (SQL).