Fix date and time

Hi domgilberto,

The way data is inputted into the cells is the main point of concern here.

Go to Control Panel >> Regional and Language Options
Here see the short date format.
This is your input format.

So if your input format is dd-mm-yyyy and you enter a date in excel like 12-31-2015 i.e. in format mm-dd-yyyy excel won't recognize it as date but as text. This is because 12-31-2015 is beyond the limits of dd-mm-yyyy, as there is no month - 31
If you enter 04-05-2015 excel would recognize it as 4 May 2015 even if you intended to input 5 April 2015.

So in your spreadsheet you have wrong "INPUT" format. Somewhere you are inputting mm-dd-yyyy and somewhere dd-mm-yyyy.
So excel is taking mm-dd-yyyy as text not dates.

The Excel FORMAT CELLS option is for changing the formats of the cell after they have been correctly inputted. So if you have inputted the date as "DATE" now you can change the format in which you want to view the date. But if you have not "INPUTTED" the date in the correct input format, excel takes it as just pure text and is unable to change the format of the cell as you want.

So basically all depends on the input. Excel would recognize a date as "DATE" only and only if it matches with the format in the "REGIONAL and LANGUAGE options".

For now I have made all your dates entered as Dates in the cells. Now excel has recognized all of them as dates and you can edit to any format you want to view the date in.
I did that by going to control panel >> Regional and Language options,
then changed my input format to mm-dd-yyyy
Then I went back to excel and all the dates which were in the format mm-dd-yyyy, I selected the cells one by one pressed F2, then pressed ENTER. Now excel recognized it as "DATE". Once all have been correctly inputted, I just selected the whole column A and formatted in DD-MM-YYYY.

I hope that helps!