Datetime def not working?

Is the datetime field not accepting the def input anymore, trying it a few ways and even with just copying from the example, it either errors, or I have to quote it out and then it just shows up as text in that field.
Trying to set the default for a date to be today-18 years so was aiming for something along moment().subtract(18, 'years')
but as I said even the function () { return new Date(); } doesn't seem to work. I went to the manual page on the inoput and don't see a def defined either.