Glad you learned something:).
Honestly don't know where exactly the 3.5 days number came from... But half the payout period would make sense as it means that when you get paid out the HBD is based on the value of HIVE in the last half of the curation window.
It was originally a week, a conservative time period that it would be difficult to manipulate the price or the median feed price over. A week was found to be overly conservative and it was halved. 3.5 days is still pretty conservative but it hasn't been changed since.
Thanks! Was the change from 7 to 3.5 done at the same time as the PD time reduction from 2 years to 13 weeks? Or was it a separate change?
Yes, hard fork 16.
https://hive.blog/steem-history/@gadrian/peeking-into-steem-s-history-hardfork-16