Question: I manage a support forum for software that changes pretty rapidly. Content therefore ages out in usefulness.

We are currently trying to figure out how to address this... and if we should. Does anyone here have best practices around labeling/archiving/removing older content? Heck, even going through and locating what is "older" content can be challenging.

Has anyone tackled this? How have you addressed it?

Answer: 
It's difficult to give a single answer as to when these times would be, but if your target audience is school students then they are more likely to be online just after school (~4pm - depending on exactly when they finish school) and at weekends, whereas if you are targeting working adults then lunchtimes and later evenings might be the best time.

If possible you should do some research to find out when your users are online and likely to see the content. If you can access logs of when users have visited the site so that's a good starting point, however, with a public platform like Facebook that won't always be possible.

After that you want to make sure that the content is going to be visible for long enough for most of your target audience to see it. On a busy site you might not have very long before other posts push yours down the page, so you need to pick your time with care. If it's really important you could consider pinning the post in some way so that it stays visible for longer.

Some sites (like Stack Exchange and Facebook) will republish a post to the home page if it changes in some way (edits, replies, comments etc.) so you could periodically prod the post in some way so that it reappears on the home page. Be careful with this though, some sites (especially Stack Exchange) take a dim view of someone making meaningless edits to their posts just to bump them.

The other thing to consider is sharing links to your post on other social media platforms at different times of the day to catch a wider spectrum of your audience.
It really depends on what kind of content you're posting (B2B or B2C) and what your CTA (call-to-action) is. 

My recommendation would be posting around 11 a.m. - 1 p.m. (when people are on their lunch and typically looking at their phones or checking their social media) or around 4 p.m. - 6 p.m. (when they're probably done with their day and are waiting for the clock to strike 5 p.m.). Typically I would post more mid-week or Friday afternoon since Mondays, people are catching up over the weekend if you're targeting businesses. 

It's really based on your demographic. I would research the typical times people are looking at their phone in your area. I live in San Francisco, so my times would be way different then someone in Indianapolis.