I have formatted cells with a date format by selecting the cells and choosing 
Format->Cells...->Numbers Tab, then selecting "Date" as the category and 
choosing a date format (for example, DD/MM/YYYY).

Then if I try to enter a date into the cell using the above format, the date is 
treated as text. It is left justified and it cannot be used in numeric formulas.

For example:
     In cell A1 enter 18/11/1997
     In cell A2 enter =A1+1

The result is:
     In cell A1, left justified 18/11/1997
     In cell A2, right justified 31/12/1899

If instead I enter in cell A1 =DATE(1997;11;18), the result is (correctly):
     In cell A1, right justified 18/11/1997
     In cell A2, right justified 19/11/1997

The same correct result is given by entering in cell A1 =35752

In a cell that is formatted as a date, I should be able to enter the date 
directly and be able to make calculations with that date.