What's the best average time zone to reach US market?

That may be true -- but I'd argue that depending on the medium, the time of day that something is posted still can play a crucial role in how well it's received. For instance, post something too early on a forum and it could be pushed to the bottom of the page by the time your target audience finds it. Send a B2B email on a Friday night and it may get buried in your target's inbox by all the other email she receives over the weekend. Same thing goes for a press release: you gotta catch reporters at the right time. And of course there's social media. On Facebook, for instance, we absolutely time our posts to get the best bang for the buck. We'll run informational stories most of the day, shopping-focused posts in the evening when people are home from work and late at night, we try to post something lighter since people are often tired and looking for escape.

As to the OP's question, it really depends on what kind of forum and who's on it. A forum for stay at home moms might see spurts of activity early in the morning between feedings, changings and dressings and later in the afternoon during naptime. Other forums and users might be best to target during business hours while others might be best to catch after business hours. But since blanket statements are impossible to make, why don't you spend 15 minutes clicking through the posts on the forum you're targeting, looking at what times people from the places you're most interested in tended to post the most?

For afterwork, I would set 05:30PM because it's called 9-5 for a reason and you have commute to home too and then first thing people do is grab a dinner with family. Assuming such I would set it 06:00PM