In Excel, I have a very long list of entries showing a date and a time in the
same cell (e.g. 27.11.2007 15:13, date is in European date format). I would
like to convert each cell to a number so I can perform calculations.

However, using the Menu: FORMAT - CELL - NUMBER does convert it to a number.

Note: If I input the date and time by hand in the same cell then formatting
as outlined above works.

Can you help?