Hi Aly,
I highlighted the cell, then selected Format > Cells and picked the date shown as 1999-01-01
This opens the formatter at the bottom of the pop-up and shows the format as YYYY-MM-DD and all I did was added a space then HH:MM:SS and this converted the cell to match the required format of 1999-01-01 00:00:00
I then used the format brush to copy that format to the rest of the date cells.
I then saved as a csv file.
All worked fine.
Hope this helps.
David