Create a custom number format

Guidelines for date and time formats

• Display days, months, and years    To display numbers as date formats (such as days, months, and years), use the following codes in a section.

m	Displays the month as a number without a leading zero.
mm	Displays the month as a number with a leading zero when appropriate.
mmm	Displays the month as an abbreviation (Jan to Dec).
mmmm	Displays the month as a full name (January to December).
mmmmm	Displays the month as a single letter (J to D).
d	Displays the day as a number without a leading zero.
dd	Displays the day as a number with a leading zero when appropriate.
ddd	Displays the day as an abbreviation (Sun to Sat).
dddd	Displays the day as a full name (Sunday to Saturday).
yy	Displays the year as a two-digit number.
yyyy	Displays the year as a four-digit number.

refer source: http://office.microsoft.com/en-001/excel-help/create-a-custom-number-format-HP010342372.aspx