Date Format not remaining as specified

I recently switched to LibreOffice. I like it so far but have a problem in calc with dates. I set the format as mm/dd/yy. In some documents it works fine, even if the date is entered as m-dd-yy, it switches it the the correct format always as it did in the previous program. In other documents it does not. I have one document with 2 different columns with date formats and it works in one and not the other. I have looked to see if there was something setup differently, but I cannot find it.
OS Windows 7.

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