How to set Date variable to null in VB.NET

Use nullable datetime

    Dim DOB As Nullable(Of Date) = Date.Now
    Dim datestring As String = DOB.Value.ToString("d")
    DOB = Nothing

“DateTime is a value type (a structure) and can not be set to null or nothing as with all .Net value types. The default value for a datetime value is zeros for the date portion and 12:00:00 AM for the Time portion.”

VB.NET - Nullable DateTime and Ternary Operator