The apostrophe is not actually present in the cell. It's just a marker that the value is text when otherwise it would be interpreted and converted by Calc into a number. Dates are just numbers (integers, actually).
The simplest way to fix it is to choose the "detect special numbers" option when you do the import ... oh, hold on, are you actually still using OO 3.0? If so, it's time to upgrade if at all possible. The text import has had some important changes since then, if I recall correctly.
If you don't see the "detect special numbers" option, or can't repeat the import, you can fix the problem in the spreadsheet by first making sure that the cells in the date column are not formatted as text (any other format is ok), then do
Edit > Find & Replace
Search for: .*
Replace with &
Options/Regular expressions: ON
Options/Current selection only: ON
Click "Replace All".
This doesn't change the text, instead it allows Calc to reconsider the imported text under conditions that allow the conversion to number values (dates).
I Hope this will helps you