Validation to keep a date field blank?

[iChuzCHRIST]

I'm stumped on this. I have a date needed fied (ec___needed). The question prior is a radiobutton called type (ec___type) with two choices (current and summer). If they choose current, I want the date field to be required. I have that working just fine. Right now, a date is not required if "summer" is chosen. What I would like to do is require that the date field be blank if they choose "summer". I know that the condition is:

return '{ec___type_raw}' == 'Summer';

However, I am not sure which validation to use, or how to set it up. I fiddled with greaterthanorlessthan, and isnot, but I just can't get it work because I don't know how to require it to stay blank.

Can anyone help with this? I can certainly keep it as it, but the OCD in me would like that to be blank if it's needed in the summer.