Jimmy ,
Yes I wanted also to use a timer, would be preferred. However NOT available to or in both Board versions and weather or not the BLE and IMU function properly with sd_power_mode_set(NRF_POWER_MODE_LOWPWR); functions AFAIK, other views are welcome.
I’m pretty sure that’s why SEEED Licensing put the RTC on the expansion board. RTC interrupt WAY more reliable. and there original If you want to call it “engineering” choices where the wrong ones. Mbed, no mbed either way it’s “HALF” a loaf. just like the battery reading shenanigans’ 
On my board I added the RTC just to overcome and enhance the capabilities.
Rev 2.0 board all mostly SMD parts, NO RTC (found out about NO low power support with BLE and IMU, or reliable battery reading.
Rev 3.1 with RTC , Serial1 port and a display interface connector and a buzzer.
HTH
GL 