Click date to add event

I have the feeling somebody must have asked about this before, and I'm pretty sure none has done it before, I just wonder why?

I'm pitching for a project that requires this - users can click on a date on the calendar to add events. I'm thinking of using Drupal as the CMS, but the lack of this feature may be the stumbling block. Basically they want something like Google Calendar, not the AJAX thingamajig, just the ability to click date to add event is good enough. If someone has implemented it before, please share. Thanks in advance!