Neither the certificate generation dialog nor the certification dialog offer this additional method for specifying the expiration date. The calendar popup of the date combo box allows to add some useful values like "1 year", "2 years", etc., which I think is sufficient.
Nobody wants to extend the lifetime of their certificate for 314 days or 42 weeks or 7 months. :-)
And if somebody wants this, then they can calculate the actual date with Excel or use gpg directly.