Where were you trying to set the date? That trick might work for an old El Capitan installer if you were doing it on a real Mac, in Terminal while booted from a USB installer, but getting to that point in a VM install could be difficult.
As a general note, if your "date" command worked, but your attempt to set the date said "command not found" then you probably typed the command wrong, e.g. omitted the required space between the word "date" and the date you were trying to set. Even if you got that right, trying to set the date that way in Terminal on a normally booted macOS isn't going to work because it requires root privileges and could have all sorts of nasty side effects on software which doesn't expect the date to be changed back several years.
I also note that your date parameter "0711141516" would be trying to set the date/time to 2016-07-11 14:15:00, not something in in 2015 as you thought. (The insane field order of MONTH DAY HOUR MINUTE YEAR doesn't help but is there because of the US convention of month/day and consistency with historic UNIX implementations.)
A better approach would be to download a new copy of the El Capitan installer from Apple, which won't have the expired certificate and therefore doesn't need the date set backwards. Apple's support page for downloading old macOS installers is here: https://support.apple.com/en-us/HT211683
There is a catch: to get the El Capitan installer application out of the downloaded disk image you must run the installer package on a Mac which is officially supported by El Capitan. If your Mac is too new (introduced in late 2016 or newer) and you don't have easy access to an older Mac, my suggested workaround is to create a macOS VM for a newer version which your Mac run, then from inside that VM, run the installer package to create the El Capitan installer application, copy the application back to the host, then use it to create your El Capitan VM. This trick works because the installer package doesn't check model compatibility if it detects it is running in a VM.
If you need more details, I've explained this in other threads in this forum previously.
