Please check are these texts which looks like dates or not. Dates are actually numbers in Excel. You may test in any empty cell as =ISTEXT(A1) if in A1 is any such date.
@Sergei, thanks for your reply, I am working on my Project Management Plan (PMP) so I'm using excel to do up my schedule to enable me generate a Gantt Chart. The problem is some of the cells worked and some don't.
Some could be texts some not and it could be at least another dozen of reasons why you have an error. Without the sample file it's hard to say what's wrong.
