You need to throw some hardware at it for what most people consider an 'off' computer. There is no hardware on the Pi for this.. likely because most people will power the Pi from mains and are fine leaving the Pi on all the time. There is the cost factor as well, however minimal it may be.
I developed a UPS that can power down and more importantly power back up the Pi at set intervals. If for example you only need your Pi to snap a picture every 15 minutes for a webcam, this will save loads of power if you are running from limited battery or solar.
Check the "UPS Timer Control Settings" on page 15 of the PDF, works like cron.
It does a fair bit of other things too.
You can roll your own on/off circuit too. Either microcontroller based or analog. It's all possible, depends if you want to spend more time and build your own or more money and buy something.