There are two closely related UnitDimensions related to temperature in the Wolfram Language: absolute temperature units and temperature difference units.
Quantity expressions that have UnitDimensions with "TemperatureUnit" represent an absolute temperature, while those with dimensions of "TemperatureDifferenceUnit" represent the difference between absolute temperatures. It is important to know that the two are related, but not directly interchangeable.
Unit conversion and multiplication are noncommutative operations for absolute temperatures, which means conversion and then multiplication will not necessarily result in the same result as multiplication and then conversion. Here is an example.
Plus will operate on the sum of temperature units and temperature difference units, and on the sum of temperature difference units, but not always on the sum of just temperature units.
A sum can also be calculated for two different absolute temperatures, if both are SI measures (i.e. have a base of kelvins):
When working with absolute temperatures, it is imperative to first standardize the units, to allow for proper unit conversions.
The single-argument form of UnitConvert will convert any temperature to kelvins: