Light day

One light day is the distance that light travels in one Earth day, a distance of 24 light hours.

In 2265, the USS Enterprise (NCC-1701) was a few light days away from Delta Vega after it had cleared the galactic barrier. 

In 2267, when Spock requested a leave on Vulcan, the diversion would have caused a loss of 2.8 light days to the course of the Enterprise.