Many places in Stack Exchange include timestamps for events, the time posts were created, commented on, edited, answered, etc.
When you 'hover' over those times, it gives a 'tooltip' indicating the exact time (in UTC) that the event occurred.
For example - hovering over the
asked 2 hours ago you get the time:
Often, when timestamps are relatively far in the past, it is very hard to mentally calculate the day-of-week.
Occasionally it is necessary (especially as a site moderator) to know whether something happened on a Wednesday, rather than a Sunday.
Adding the day-of-week to the tool-tip would be a simple change but would add a significant amount of value:
Mon 2015-01-26 09:22:17Z