Dates and times are displayed in many places. It's important to have a consistent, readable standard for how they're presented.
Relative time calculations should be used in cases where it's important for a user to know how old something is, or how long ago something happened. An example is last saved times. It's much harder to tell at a glance how long ago something was saved if it was an absolute date and time, so consider instead using a relative date.
Last Saved 4 minutes ago
Absolute dates and times should be used when it's important for the user to understand when something happened, as opposed to how long ago. Accuracy here can be much higher than a relative date. For example, change history should show an absolute date since users want to know exactly when an event happened.
Mar 10, 2015
It's important to consider when and when not to show the time along with the date. To avoid visual clutter you may want to show the date only and omit showing the time. However, if the time is essential for the user, show the time along with the date, do so.
Mar 10 2015, at 9:54 am
You should avoid showing only the time for when something happened. In most cases the date is also important to the user. In the case that all information in a given page is all the same day, or the user has filtered a list of items to a particular day, it may be appropriate to omit the date.