If you set a timer to start when you click on the buy, you don't have to worry about time zones. The timer will do it's own thing. So actually, you could even put it on for however long the booster is, if it's a month, week, however long it lasts.
The only issue I see is that you have to pay for things with money, on Steam. I'm sure they'd have a way to do it though, based on client. Not time zone.