You can create a Calendar object that stores a single selected date from a specific period (use Min / Max fields to set the start and end of your calendar) in a variable. Variable values will survive a reload (if you do not modify them in your script) so if you format your vdate value based on the variable value from your calendar, you will be fine.
Most databases accept date values in database format i.e. a string formatted as 'YYYY-MM-DD'. If we assume a calendar variable called vCalendarDate, your code could look like:
LET vdate = text(Date(Date#(vCalendarDate), 'YYYY-MM-DD'));