If you want to use a format that isn’t in the Type box, you can create your own. The easiest way to do this is to start from a format this is close to what you want.
Select the cells you want to format.
In the Format Cells box, click the Number tab.
In the Category list, click Date, and then choose a date format you want in Type. You can adjust this format in the last step below.
Go back to the Category list, and choose Custom. Under Type, you’ll see the format code for the date format you chose in the previous step. The built-in date format can’t be changed, so don’t worry about messing it up. The changes you make will only apply to the custom format you’re creating.
In the Type box, make the changes you want using code from the table below.
You will find more informations in the upper link.