I've been trying to figure what's the best way to calculate duration of an event.

I have a Collection with the following fields:

```
type Event {
 ..
 startsAt: "HH:mm MM-DD-YYYY"
 endsAt: "HH:mm MM-DD-YYYY"
 ..
}

```

Ideally I would like to save 

```
duration: String!

```

in my GraphQL mutation, when creating an Event.

Could someone advise me how could this be done?

**My stack:** 

* React
* Apollo
* GraphQL

**I have the following installed:**  

 .. 

* MomentJS
* React Moment

..

Many thanks in advance