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The HardFork is scheduled for Tue, 27 August 2019 15:00:00 UTC, so if someone create a proposal just after it and vote with power, then they have a chance to grab all rewards from first @steem.dao that is going to happen at 16:00:00 UTC.
But as I said, because SteemDAO starts empty, payouts at the beginning will be very, very small.

So, yes, the window is 24 hours on the utc?
That seems to disadvantage the less active.

Being first isn't worth the 10sbd, for me.

No, payout is every hour.
I don't think that fighting for being first is worth for anyone.
Like I said, first payouts will be pennies if any.