Anyone who has had a min/max thermometer for more than about three days figures out that you have to reset the thermometer at night. Otherwise, half of your readings are going to be incorrect.
For instance, if day one had a high temperature of 72 degrees at 3PM, the observer took their reading every day at 3PM, and a cold front came through that night – day two would also record a bogus high temperature of 72 degrees.
I figured this out when I was seven years old. The largest GISS/USHCN post 2000 adjustment is based on the idea that the observers are all morons (like the people making the adjustments.) It also assumes that a few climate activists can deduce the day to day behavior of tens of thousands of observers over the last two centuries.