Is it possible to leave a date field blank/empty?

I am using a date picker.  For some of my fields it is nice that it defaults to the current dates, but sometimes I want the option to leave it blank.  For example, "When was the last time it rained over 0.5 inch?".  If the user skips this questions, it defaults to today's date.

You can only either make it default to todays date or leave it blank. You can do that by setting the Default Time to on or off.