This branch will totally re-work and simplify the Views filters. These changes may/will break some views and will require that you check views that use the date filter or argument to be sure they are still configured correctly.
There is a new date_filter_handler_simple for an individual date field that simplifies the Views handling by using most of the core Views field handling but adds in the possibility to set a default date using options like 'now', along with a choice of a month/day/year drop-down selector or popup calendar selector.
The date_filter_handler is an extension of the simple handler that allows you to combine multiple dates into a single filter so that you can control all of them with the same filter widget, allowing you to either 'OR' or 'AND' them together.
The arguments will be rewritten in the same way, with a simple argument handler for individual dates and another handler that extends that to control multiple dates with a single argument.
The Calendar module will need an update when this is finished to use the new handlers correctly.
This code is NOT yet ready to use.