Periodic waking up from deep sleep
Generally an RTC interrupt is the best way to go for a long period in deep sleep. If you can use the RTC built into the micro this will generally give you a simpler implementation but may not be the absolute lowest power possible or most accurate.

If you do go the mbed route then this library will probably be of use. However take care, some of the boards are not really well designed for lower power, while the processor will go into deep sleep on just about all of them some of the boards can't shut the rest of the circuits into low power modes and so the total system deep sleep power draw can be far higher that you'd hope for. Other boards are better able to cope with this and can get down to tiny power consumptions.