It takes a pretty long time to enter clients' birth and anniversary dates by use of the calendar. Can't I just enter those manually?

The calendar to the right side of the Birthday and Anniversary fields is only an alternative option. You can click on each field and type in the dates, ignoring the calendar pop-up. Make sure you use the correct date format: MM/DD/YYYY. In order to save, it's enough to press enter after typing in the dates. Or you can save by scrolling down and clicking on the "Save" button.