Many countries changed from Julian to Gregorian calendar between 16th and 19th century.

When they did the switch - they skipped about 13 days (depends on when exactly they did it, for some it was 12 days).

And yes - the recorded monthly/quarterly/annual and hence min/max/avg temperature that we can find online [1] for the year particlar country switched is 0, NaN, YOLO, or maybe even negative ...

Hence combination of "average earth temperature" intially being set by just handfull of countries/records/rows would make you think that there was a small ice age, when it was just Christians (re)doing date math to catch up with actually natural seasons

PS [1] wife did some data analysis training during Covid lockdowns, students were given links to github "reference" annual temperature data and some outliers caught our attention