Date formats

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Hello,

friends...
i have problem in the date formats
i have an excel sheet with data
i am trying to filter by date
01/12/1999
this is the date format - dd/mm/yyyy
but some data takes 01 as day and for some data it takes as month
so when filtering by the date wise.. i am not getting correct data
how to set this problem
how to make all the cells as one format
while changing the date format.. its not working