DualDatePickerDialog can select a date from one of two calendars. In many countries optional calendar is used together with main calendar. For example in Korea, Gregorian calendar is used mainly but Korean lunisolar calendar is also used for various traditional events. DualDatePickerDialog lets the caller to select a date from one of two calendars by showing them altogether. User can switch two calendars using UI elements (which will be changed to improve usability). DualDatePickerDialog uses CalendarDatePicker which is not Android standard DatePicker widget. CalendarDatePicker similar to android's DatePicker widget, but works with Calendar instance. Android's DatePicker widget works well with GregorianCalendar, but doens't support non-Gregorian calendar. CalendarDatePicker works with any Calendar-subclassing instances. This package provides a KoreanLunisolarCalendar class to support Korean lunisolar calendar.