[verletx64]

You weren't born on a range of days.

You're unlikely to change your mind about when you were born.

You really only ever need to provide three well defined numbers.

So, why display a calendar?

[PeterisP]

Well, one issue is that there are different date formats used worldwide and there's a risk that the website will accidentally swap the month and date if I simply enter three numbers without thinking about them. mm-dd-yy and dd-mm-yy are both used occasionally.

[Blaiz0r]

There are also different calendar formats.

https://msdn.microsoft.com/en-us/library/cc194816.aspx