Dates using "/" (slash) changing into "-" (hyphen)

I have 2 serious problems.

1. When i format dates using "/" (slash) in my system and open the same excel file in another system the separator is changing into "-" (hyphen). This is seriously effecting my project deadlines.

2. I format the dates into yyyy/mm/dd but when i open the file in other system the format is changing into yy/mm/dd. How can i keep my formats same in all systems.

We have Office 2007 installed in all systems.

Plz Help.

Rgds