Observation times for the Cooperative Observer Network (COOP) have not been digitized for observations taken before 1982, yet they are needed for various climatological applications. Observation times were estimated to assist with quality control of NCDC’s new Summary of the Day TD-3206 data set. The estimated observation times are based on the correlation of the maximum temperatures for a station with the surrounding stations on a monthly basis. Estimated observation times are distinguished as either AM or PM, with midnight-observing stations falling in the PM category. The observation time estimation technique was developed and tuned using data from Illinois stations. The technique starts with a set of known (digitized) observation times for a set of Illinois stations. The observation times are then estimated for other continental U.S. stations in order of increasing distance from Urbana, Illinois. The technique identifies AM observing stations with maximum temperatures digitized shifted back one day as PM stations. For the period 1898-1947, the average number of digitized COOP stations with AM observing times digitized as such was 10% for the continental U.S.; the number of stations with PM observing times and AM stations digitized shifted back one day was 90%. Application of a technique for identifying AM-shifter stations suggested that 75 stations have at least three years of AM-shifted maximum temperatures. These stations were not evenly distributed across the U.S. The estimated observation times indicate over 130 stations switched between AM observation times in the summer months and PM observation times in the winter months for at least one year. The observation time estimation technique also distinguished individual months for some AM observing stations in which the editor failed to mark the shifting of the maximum temperatures back one day, resulting in stray months digitized "correctly" as AM in a series of months otherwise digitized shifted to "PM".