The [Timeago][1] module (available for both D6 and D7) can be used for this. Here is an excerpt from its project page:

> Uses the jQuery [timeago][2] plugin to create dynamically updating "time ago" dates. That is, the plugin turns static dates like "October 10, 2011" into "10 minutes ago" and updates the time ago every minute. This allows you to include "time ago" dates in cached content for most users while degrading gracefully for users with JavaScript disabled.

> Features included:
> 
> - An option to turn node created times into timeago dates
> - An option to turn comment created/changed times into timeago dates
> - Tokens for node created time and comment created/changed times
> - A timeago date formatter option for use with dates anywhere
> - An API to turn any UNIX timestamp into a timeago date

Obviously, in this case you're looking for the feature mentioned in the first bullet above.

  [1]: https://www.drupal.org/project/timeago
  [2]: http://timeago.yarp.com/