This site's own guarantee section states this directly: calendar your refund deadline as soon as your order arrives so you do not lose track of the window.
The 60-day window runs from the original purchase date — worth marking that exact date immediately on delivery, rather than losing track of how much of the window has already passed.
Set a calendar reminder the same day your order arrives, and confirm the exact current wording of the guarantee terms on the official checkout page before ordering, since this site notes terms can change.