Date Format in the United States
The United States is one of the few countries that use “mm-dd-yyyy” as their date format–which is very very unique! The day is written first and the year last in most countries (dd-mm-yyyy) and some nations, such as Iran, Korea, and China, write the year first and the day last (yyyy-mm-dd). But why did Americans choose to write the month first? One of the hypotheses is that the United States borrowed the way it was written from the United Kingdom who used it before the 20th century and then later changed it to match Europe (dd-mm-yyyy). American colonists liked their original format and it’s been that way ever since.
The United States has a rather unique way of writing the date that is imitated in very few other countries (although Canada and Belize do also use the form). In America, the date is formally written in month/day/year form. Thus, “January 1, 2011” is widely considered to be correct. In formal usage, it is not appropriate to omit the year, or to use a purely numerical form of the date. For example, if you were to write a formal letter for business, you would write out the entire date, including the name of the month (January 1, 2011). Writing it out in full allows for the notation to be understood even by people for whom the month/day/year form is relatively uncommon.— grammar.yourdictionary.com