Migration from vba to vb.net - correct logic for using Iserror

I'm trying to use this code:

If Not IsError(Convert.ToDateTime(DatiBase(i).innertext.substring(DatiBase(i).innertext.indexof(":") + 1).trim(), New CultureInfo("it-IT"))) Then
    MyString = "'" & Convert.ToDateTime(DatiBase(i).innertext.substring(DatiBase(i).innertext.indexof(":") + 1).trim(), New CultureInfo("it-IT")).ToString("yyyy-MM-dd") & "'"
Else
    'Do something else
End If

My goal is to assign value to "MyString" only if "Convert.ToDateTime" gives a valid value but the code doesn't do that: It stops when the String isn't recognized as a valid DateTime instead of execute the "else" code.