Dim mylist = (From el in Element Select el).ToList()
     .Select(function(el) new with {
           .bday = el.birthday.toString("dd/MM/yy")
           }

.bday = el.birthday.toString("dd/MM/yy")

.bday = CDate(el.birthday).toString("dd/MM/yy")

Dim normalDate As Date = Now
Dim nullableDate As Nullable(Of Date) = normalDate

Dim normalToText As String = normalDate.ToString("dd/MM/yy") 'Works perfectly

Dim nullableToText As String = nullableDate.ToString("dd/MM/yy") 'Error

.bday = el.birthday.value.toString("dd/MM/yy")

If el.birthday.HasValue Then
   .bday = el.birthday.toString("dd/MM/yy")
End If