[Solved] add 20 days to start date of subscription

Sara Ree Asks: add 20 days to start date of subscription
After callback from pay service (when user has paid the new subscription) I want to save his new subscription to his/her account in DB. Using Mongoose I want to save new subscription start and end Dates in the user account like this:

Code:
const start = Date.now();

const end = ?

const subscription = { 
    start: start,
    end: end,
}

account.subscriptions.push(subscription);

await account.save();

I can get the subscription start date using const start = Date.now(); But how can I calculate the end date of subscription if this is a 20 days package?

I should add 20 days to start date how can I do this?

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