Sorry for my late reply, here is the documentation of the calculate. Basically if your worktime is the number of seconds then you can use like above like this:
"calculate"=>array(
"{sumTime}"=>array("sum","worktime"),
),
"bottom"=>function($calculated_results){
//In here you can get $calculated_results["{sumTime}"] in number
//You convert it to the H:i here.
// Later you return the text
return "Sum time:". $calculated_results["{sumTime}"];
},
As you can see that, we do the conversion in the template "bottom", there is another template is the "top".
Second option is that you may define a function for {sumTime}
"calculate"=>array(
"{sumTime}"=>function($store){
//You can return the sum of working time here or you can get the sum then further formatting the sum and return.
return $store->sum("worktime");
},
),
It work the same, however you may format the sum of working time here to format "H:i" and return it. The result can be used in "bottom" and "top" template immediately.
Hope that my answer helps. Let me know if you need further assistance.