How are free trial days left calculated by shopify?

[sharanbrar]

I have created a 7 day free trial for new app users and want to show inside the app how many days of free trial are left. 
I reduce the number of days after every 24 hours from when the app was installed, but have noticed that shopify calculates the days left in a different way and sometimes reduces the days left earlier than I do, which causes confusion for the users.
Does anyone understands how shopify calculates the days left?

[bburke4]

I just check the 

"trial_ends_on"=>"2021-01-01",

field in the RecurringApplicationCharge response object. I believe it is set when the charge is activated