This site states this specific wording directly: contact support within 60 days of your shipping date.
Shipping date and purchase date can differ by several days depending on processing time — worth marking your actual shipping-confirmation date specifically, rather than the date you placed the order, when tracking your guarantee window.
Check your shipping-confirmation email for the specific shipping date, and calendar your refund deadline from that date rather than your order date, to avoid miscounting the 60-day window.