A calendar is good for a 'free choice'. It can provide context(i.e. day of week) and can be overlayed with useful information(price for a date/availability etc.

A calendar is not good for pre-determined dates where the date is fixed before hand - i.e. you don't get to choose what value you want the date to be.

For example, if you're entering in your driving license details for insurance quote, you don't get to choose when it expires - that's already decided for you. Ditto for DOB and other such things.

So, there are far more efficient options for date entry ranging from single text input (with appropriate parsing/validation) or the '3 dropdowns option(D,M,Y)' which is more effort but easier to avoid mistakes with.

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?

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.

There are also different calendar formats.

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

For starters, the default date is generally off by anywhere from 20 to 70 years.

Isn't the most common date entry field one where you can type directly into, but have the option to use the calendar widget?