Try this:
bone$ cd /sys/class/leds/beaglebone:green:usr0
bone$ echo timer > trigger
bone$ ls -ls
0 -rw-rw-r-- 1 root gpio 4096 Aug 31 10:10 brightness
0 -rw-r–r-- 1 root root 4096 Sep 4 08:18 delay_off
0 -rw-r–r-- 1 root root 4096 Sep 4 08:18 delay_on
0 lrwxrwxrwx 1 root gpio 0 Aug 31 10:10 device → …/…/…/leds
0 -rw-rw-r-- 1 root gpio 4096 Aug 31 10:10 max_brightness
0 drwxrwxr-x 2 root gpio 0 Aug 31 10:10 power
The newly created delay_off and delay_on are not in group gpio.
–Mark