Unable to use operands with command substitution

I'm trying to do the following:

``` clojure
(set! FILENAME @(date +(quoted "foo_%Y_%m_%d.pg")))
```

The output I want is:

``` bash
FILENAME=$(date +"foo_%Y_%m_%d.pg")
```

Instead I'm getting:

``` bash
FILENAME=$(date + "foo_%Y_%m_%d.pg") # invalid syntax...
```
