Both the 15 and 20 minute intervals are dividable by five minutes. Declare a variable outside of the for loop and keep adding five minutes every iteration, test if the minutes are dividable by 15 or 20 and, if true, push them to expected time.
let x = new Date('2100-01-05T08:00:00')
var expectedTime = []
for (let i = 0; i < 10; i++) {
var m = x.getMinutes();
if (m % 15 === 0 || m % 20 === 0) expectedTime.push(x.toString().split(' ')[4]);
x.setMinutes(x.getMinutes() + 5);
}
console.log(expectedTime);