[sloxy]

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.