Book them two days before they expire.
Have you ever had your miles expire before the expiration date? This can actually happen if you are placing your miles in an foreign frequent flyer program like Asiana Airlines or Air France.
Recently, I had 62,903 miles stockpiled in Asiana, and the time had come: I had reached the magical expiration date of seven years. (It's nearly impossible to extend mileage with their program.)
Exactly 21,415 miles were set to expire January 1, and like the good procrastinator I am, I made a mental note to book my ticket on December 31.
I logged in the morning, and they were there. In the afternoon, I logged in and they were gone. They had expired.
They had expired because, apparently, Asiana miles expire on Korea Standard Time; these miles do not expire in the time zone where the frequent flyer holder is based.
Personally, in my mind, it's completely bonkers.
Most airlines, are to some effect, international companies and deal with time zone issues on an hourly basis (especially an airline that flies pretty much around the clock!). It is the actual business.
Just an FYI, and then I had a glass of wine afterwards to remind myself this what happens when you don't use them. 🍷
