A really simple question but can anyone tell me which timezone Steem runs in please?

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The blockchain operates in all timezones. It is live 24 hours a day, 7 days a week. The 7 day payout period is calculated based on when you write the post. The timezone doesn't matter. 

Steem's cashout window is calculated in seconds. Here's the line of config file that defines it:

>  #define STEEM_CASHOUT_WINDOW_SECONDS          (60*60*24*7)  /// 7 days 

> Config file line 56: https://github.com/steemit/steem/blob/master/libraries/protocol/include/steem/protocol/config.hpp

60*60 = 3600 seconds or 1 hour.

60*60*24 =  86400 seconds or 1 day.

 60*60*24*7 =  604800 seconds or 7 days.

That means, you receive your payout exactly 604800 seconds after you write the post.