Since tempered buff is available 12 hours after you buy it, how does it exactly work? I have 2 ideas:
1) It gives you a bonus depending on the time iron banner event started. The longer iron banner event has passed, the better the bonus. This means that buying the buff everytime you can is the way to go.
2) It gives you a bonus depending on the time it has not been bought from the vendor. When buying the buff, it has greater effects when it stayed with the vendor for long periods of time. This means that leaving the buff alone for now, then buying it sometime later (like Sunday or Monday) would yield better results.
So how do you think the tempered buff works? Feel free to post your ideas on how it works.