Variable problem with Time.Format

Trying to test out the new Time.Format in Hugo 87, but not able to access that from the config file.

I have put the following in both en and fr in my config and I can print the values of those settings:

    dateFormat: ":date_short"                                                                                                                    
      dateFormat: ":date_long"

And in my layout I’ve been testing:

      {{ printf "%#v" .Site.Params.dateFormat }}                                                                                         
      {{ printf "%#v" .Site.Params.params.dateFormat }}                                                                                          
      {{ $datestr := .Site.Params.dateFormat }}                                                                                          
      {{ printf "%#v" $datestr }}  < prints string in page
      {{ $datestr := .Site.Params.params.dateFormat }}                                                                                   
      {{ printf "%#v" $datestr }}  < prints string in page                                                                                                       
      {{ $datestr := .Site.Params.params.dateFormat }}   <  doesn't work                                                                           
      {{ $datestr := ":date_long" }}        < works                                                            
      {{ with .Date | time.Format $datestr }}                                                                                            
        {{ i18n "postedOnDate" . }}                                                                                                      
        {{ end }}                                

If I set $datestr directly to “:date_long” the {{ with }} section above runs fine and prints the date in the appropriate format for the language. But if i set $datestr to any of the .Site.paramaters (which print fine), then I get an error message:

"partials/post-meta.html" at <$datestr>: invalid value; expected string

What am I doing wrong here??? Thanks!