Do you happen to be using variables to select a from/to date range? Then when you clear the date field, you want your variables cleared? If so, I'll point you to a way to do it within the data model.
Subroutine to Create Data Model for From/To Date Selection
If not, you can set a variable as an action, so you could clear it that way, but I'm not sure about the trigger itself. You can trigger when changing the field, but I'm not sure of a way to only trigger when clearing a field. I guess you'd trigger every time the field changed, and set the value to an if, like if(field is cleared, blank, current value). I know that's short on detail. OK, I built an example. I don't know what you're doing with the field and variable, but in this case, triggers try to keep the two in sync, at least if there's a single value or cleared. So if you clear the field, the variable clears.
