If the timer value in ngx.timer.at() is too high, callback triggers immediately

• The exact version of the related software, including but not limited to the OpenResty version (if any), the NGINX core version, the ngx_lua module version,
  and your operating system version.

nginx version: openresty/1.11.2.5
built by gcc 6.2.0 (GCC)
ngx_lua-0.10.10
Linux 4.13.2

• A minimal and standalone test case that others can easily run on their side and
  reproduce the issue you are seeing.

http {
    server {
        listen 5309;
        location = / {
            content_by_lua_block {
                local function fn()
                    ngx.log(ngx.ERR, "Hello from callback")
                end
                ngx.timer.at(1e16, fn)
                return 200;
            }
        }
    }
}

Once I hit the endpoint, "Hello from callback" shows up immediately in the logs. If I lower the timer to 1e15, it doesn't.

Start with the standalone nginx configuration and the ngx.timer.at entry point, reproducing the contrast between delays of 1e15 and 1e16 on the reported OpenResty and ngx_lua versions. Trace how the timer delay is handled and define the expected behavior for an excessively large value; done means the callback no longer fires immediately, with a regression test covering the boundary.