You must not have read the ngx_http_ssi_module documentation properly (especially its 'SSI Commands' section): it explains the commands format.
You need to set the ssi directive to on in the context you wish SSI commands to be parsed, then you need to serve files there which contains those commands.
For example:
server {
listen 8000;
index index.txt;
location / {
ssi on;
}
}
The $date_local variable states that it must be configured with the config command, by settings its timefmt parameter.
You just need to serve files which will send commands back, such as index.txt:
<!--# config timefmt="%A, %d-%b-%Y %H:%M:%S %Z" -->
The format being used by the timefmt parameter is the one of the strftime standard C function (as the documentation states).