Validating Auto Generated Code For 24 Hours?

I am new to vb.net. I created a vb.net script to autogenerate a code. Now, after generating this code, what I want is to determine the validity of this code. for example, a code has been generated now and this code will be valid for 24 hours only.

Below is my script for the auto generation code
Dim randomvalue As New Random
Dim randomhold As Integer
Dim newcode As Long
For i As Integer = 0 To 9999
randomhold = randomvalue.Next(1, 9999)
newcode = randomhold & DateTime.Now.Minute & DateTime.Now.Year
Label3.Text = newcode
Next
The code generated will be sent via sms. If the code hasn't used within 24 hours, it will be invalid.