when you import data from some external source and end up with numbers or dates in text format (withe the apostrophe as first character) I see no way to convert the cell contents to a different format except manually removing the '. sure there are some macros "out there" or you should have checked the data type during import. but sometimes you are in that situation and a nice way out would be nice.
There is an official extension that works: http://extensions.libreoffice.org/extension-center/ct2n-convert-text-to-number-and-dates *** This bug has been marked as a duplicate of bug 33962 ***
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