I would like to know about the time format which is used by Facebook - relative times such as "2 hours ago", rather than absolute times such as "13:35".
Is there any research about which time format is easier to read for humans and why?
I'm not sure about hard research, but there's a good article from UX Movement:
When to use absolute timestamps:
There are times when users need to look back on past content to retrieve information. Without absolute timestamps, users can’t target a specific period to find the information.
Sites that host photos, documents, messages, tasks and events all need absolute timestamps. These content will hold utility in the future when users need to reference them. Use absolute timestamps when users can go back and make use of past content.
When to use relative timestamps:
Accuracy isn’t important with relative timestamps, but immediacy is. When users want to know how long ago a site published a post, they prefer time units in written form. This way they don’t have to mentally calculate dates and times and count back from the present day.
Not only that, but users don’t have to convert time zones with relative timestamps. Content published an hour ago or five hours ago makes no difference to users who only need a general sense of recency.
When you use absolute timestamps, you need to find out which timezone the user is in. This can be a guess based on IP address (which may be wrong) or a setting in the user profile (which needs to be filled out during user signup, adding another field), or a mix between both.
It is confusing to the user when you get it wrong. When I travel abroad, some websites show timestamps in my home timezone and others use the one where I currently am. The daylight savings switch in autumn will also show timestamps between 2AM and 3AM seemingly out of order, or imply that only a few minutes passed between two entries, when it was really an hour.
StackOverflow shows relative time for recent posts and switches to absolute (date only) after some time, when no one cares about the time of day a post was made anymore. What is best for your website depends on your application, whether there will be access to old material, whether users need to follow nonlinear reply chains, ...
The choice between absolute (13:35) and relative (2 hours ago) is made depending on the type of content you're browsing. Absolute timestamps are used when your user needs to reference something; sites that host photos, documents, messages, tasks and events all need absolute timestamps. You need to know the exact date something was published, it's not important to know how long ago that was from where you are now.
Relative timestamps are more emotion oriented; when your site content updates often, or has a lot of user activity, you use relative timestamps. This is important because you are trying to see whether or not something is new compared to where you are now. A stamp saying 2 hours ago is much faster at telling you this is fresh content. You don't have to mentally calculate dates and times and count back from the current time. This also avoids the whole timezone issue.