KMS CS item that extends equip expiration

No, not exactly. You can not extend you item past a 30 day expiration date. So let's say I extend an item that was going to expire in 1 day to expire in 21 days. I now cannot use another 20 day hourglass on that item. But, once the expiration date reaches 10 days I could use another hourglass and bring the expiration date up to 30 days, but the next hourglass wouldn't be able to be used until 20 days later. So, you can extend however times you want, assuming that the extension will not exceed a 30 day extension at any one time.

Then how do 50 and 100 day hourglasses work?