You can find a calender control on the Toolbox by clicking the last icon
("more controls") and then select the "Calender Control 9.0".
You name this calender page to e.g. "mydate", then you can refer to the
date-value by me.mydate on that form. To make it pop up, you need to make a
dedicated form that pops up when you enter a date field in some other form.