Ignore Maths, and DateDiff: use a
TimeSpan[
^]
You are mixing up very old VB (VB6 and earlier!) constructs with the newer .NET versions, which is never a good idea.
If you subtract two DateTime values, you get a Timespan:
Dim dt As DateTime = New DateTime(2022, 7, 15)
Dim diff As TimeSpan = dt - DateTime.Now
Console.WriteLine(diff)
The TimeSpan struct includes a Duration method, which returns the absolute value of a Timespan:
Dim dt As DateTime = New DateTime(2022, 7, 15)
Dim diff As TimeSpan = (dt - DateTime.Now).Duration()
Console.WriteLine(diff)