  The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) has
produced a large volume of high-quality lunar laser ranging (LLR) data since it
began operating in 2006. For most of this period, APOLLO has relied on a
GPS-disciplined, high-stability quartz oscillator as its frequency and time
standard. The recent addition of a cesium clock as part of a timing calibration
system initiated a comparison campaign between the two clocks. This has allowed
correction of APOLLO range measurements--called normal points--during the
overlap period, but also revealed a mechanism to correct for systematic range
offsets due to clock errors in historical APOLLO data. Drift of the GPS clock
on ~1000 s timescales contributed typically 2.5 mm of range error to APOLLO
measurements, and we find that this may be reduced to ~1.6 mm on average. We
present here a characterization of APOLLO clock errors, the method by which we
correct historical data, and the resulting statistics.
