Scheduler Custom Command not accepting certain characters

[scholtron]

I'm attempting to backup the configuration of my router via the Scheduler using the Custom Command function.

To replicate the issue go to Administration > Scheduler, enable Custom 1 and paste the following into the Command text box and hit save:
nvram save /tmp/router_config_$(date "+%Y-%m-%d").cfg

Refresh the page and the command should be modified from the originally pasted command, it becomes:
nvram save /tmp/router_config_$(date "+++).cfg

To boil the issue down further, using this simplified date command:
date +%F

Turns to:
date +�F

Something about the percent (%) symbol throws things off. There's a couple of workarounds I could think of, but I used the following command within the Scheduler (rather than using a script on the filesystem):
nvram save /tmp/router_config_$(date -I).cfg

Just reporting in case any others stumble upon a similar issue.