Creating calculations in app from insightly data

Hi, I'm brand new to Power BI and am still learning what I can do. I've connected to insightly, mailchimp and Google Analytics and have the premade dashboards, which is great. But, now, I'd like to create my own reports. I can filter data in the Power BI app but am I able to create calculations in there? For example, I'd like to know the difference between two fields: Date_Created and Forecast_Close_Date (both from insightly) so I have a better understanding of how long it takes my sales team to close an Opportunity. I can't do it in Desktop because insightly isn't available there yet - how would I do it in the app?