If you’re using a Twitter client such as HootSuite, you can easily see the exact time that a tweet in your feed was posted, as HootSuite conveniently displays the timestamp above the message. Other clients work in a similar way. On Twitter.com things are a little different. There, Twitter prefers to show you how long it was since a tweet was posted. For example, it might say 55 seconds ago, 19 minutes ago, or 1 or more hours ago.

At one or more hours, Twitter.com tends to clump tweet timestamps together, so you get many tweets showing the same timestamp (i.e., 2 hours ago), even if they were posted at very different points over that hour. And when you move to a day or more, Twitter.com displays 1 day ago, or 5 days ago, but doesn’t tell you anything about the time at all.

So how do you find out?

It’s actually incredibly simple – all you have to do is place your mouse pointer over the timestamp on Twitter.com, either in your feed or within the desired tweet, and Twitter will show you the time the tweet was posted via a little pop-up.

As an example, here’s a tweet captured from our feed earlier today.

Notice it says that it was posted 5 hours ago. That lets you make a rough estimate of the time it was posted, but that’s about it. However, if you mouseover the timestamp part of the tweet you can see the exact time.

11.04 AM, on October 7, 2011.

Note that this time is local time to you, depending on your Twitter timezone settings. So, for me, that’s 11:04 AM in the United Kingdom. If you’re in New York, the timestamp will show 6:04 AM.

Why is this useful? Well, sometimes you’ll want to know exactly when a tweet was posted, certainly if you’re tracing the chain of events of an important story. It can also help you determine who was the first person to break a story on any given day… and who is claiming credit for somebody else’s work.

This works with tweets of any age. Here is the first-ever tweet, written by Jack Dorsey, dated March 21, 2006.

Note that when tweets go back this far Twitter does put a full date on them. But we don’t see the time. So, again, mouseover the date, and the time the tweet was posted will appear.

There you have it: 8:50 PM. Again, this is in my local time. We know that Twitter (and Jack) are based in San Francisco, which is PDT. PDT is GMT minus 8 hours, which means that Jack wrote the first-ever tweet in San Francisco at 12:08 PM PDT on March 21, 2006.

Which, incidentally, was a Tuesday. Although Twitter won’t be able to help you with that.