Observing logs are filed by month. During January and February 1999, observing logs are kept by UT date. This caused some confusion, as calibration images for a given night are in the logs for the previous UT date. Not a good situation.
To address this "detachment" problem, we instituted a new dating format on 1999 February 28. Logs are now organized by "Observing Date", which is defined as the local date at the start of the observing night. In this system, to find the observing logs for the night that began at sunset on 1999 June 12 and ended at sunrise on June 13, you would look at the log for 1999 June 12.