cron job for rtcwake

I’m trying to configure the beaglebone black [wireless version - 4.9.82-ti-r102 #1 SMP PREEMPT] – running debian 9.3 stretch.

This command works fine in a bash terminal:

sudo /usr/sbin/rtcwake -m mem -u -t $(date +%s -d "+2 minutes")

I’ve setup a cron job via

sudo crontab -e

In it, I have the following line:

10,40 * * * * /usr/sbin/rtcwake -m mem -u -t $(date +%s -d "20 minutes")

However, the device is not suspending.

What am I doing wrong? How do I debug this?

Thanks for your help