What is the applicable design pattern for a datepicker widget?

What is the design pattern for a datepicker widget? jQuery sample follows.

At this point I'm thinking Observer Pattern? A click on the input field generates an event. Observer listens for this event, and when triggered, displays the calendar for picking the date.

I have nagging doubts about this assessment; I don't feel like I have it right. An observer is really for a 1:many relationship; this is 1:1.

Thanks for your help. Keith :^)

Example of datepicker widget from jQuery UI:
http://ift.tt/1sUhvPK