The following code works:

let birthdate = Date(timeIntervalSince1970: 0)
let retireAge = 65

var retireDate: Date {
    let retireDate = birthdate + (65 * 31536000)
    return retireDate
}
retireDate

When I substitute "65" with "retireAge" witin the retireDate computed property like so....

let birthdate = Date(timeIntervalSince1970: 0)
let retireAge = 65

var retireDate: Date {
    let retireDate = birthdate + (retireAge * 31536000)
    return retireDate
}
retireDate

...I get the following error: "Binary operator '+' cannot be applied to operands of type 'Date' and 'Int'"

I'm not sure why this error occurs or how I can work around it.

Thought @roosterboy would get to this. The way you are doing it does not take in the account leap years etc so you are always better using the Date functions.

But I suppose you asking also why when you changed 65 to retireAge, why 65 worked but retireAge did not!

It because that the systems infered 65 as the first was date. it the same as

var double = 2.0
var int = 1

let answer1 = double + 1
let answer2 = double + int // <- this does not work

Specifically, in the first case:

let birthdate = Date(timeIntervalSince1970: 0)
let retireAge = 65

var retireDate: Date {
    let retireDate = birthdate + (65 * 31536000)
    return retireDate
}
retireDate

Swift's Date type has an operator + which works on a Date and a TimeInterval. And TimeInterval is just a typealias (i.e., another name) for a Double. 65 and 31536000 are inferred by the compiler to be TimeIntervals (a.k.a. Doubles, technically as 65.0 and 31536000.0). So adding a Date and a TimeInterval (a.k.a. Double) works.

Essentially, you have:

Date + (TimeInterval * TimeInterval)

But in the second case:

let birthdate = Date(timeIntervalSince1970: 0)
let retireAge = 65

var retireDate: Date {
    let retireDate = birthdate + (retireAge * 31536000)
    return retireDate
}
retireDate

You declare retireAge as an Int using the literal 65. So using it in the date math line won't work because what you have is:

Date + (Int * TimeInterval)

and you just can't mix types like that (at least not in this case).

Thank you! All three of your responses were extraordinarily helpful!