I have done a custom board design using the SAMD21. In the design I use the RTC to wake the processor every minute, and then put back to sleep.
In sleep I am consuming ~50uA with the RTC running.
There quite a few things I had to do get down this low, for example make sure all GPIOs are not floating and turning off the peripherals and clocks not used. The code was for a contract I did so I can not share but will try to get an example posted if it would help.
Trampas
misfittech.net