#DeltaT# just means a change in temperature.
You calculate #DeltaT# by using the final temperature and the initial temperature of something. More specifically, you get #DeltaT# by subtracting the initial temperature from the final temperature.
#DeltaT = underbrace(T_f)_(color(blue)("final temp")) - underbrace(T_i)_(color(green)("initial temp"))#
This means that, if your two temperature are expressed in Kelvin, #DeltaT# will also be expressed in Kelvin. For example, if you have some water at #"293.15 K"# and you heat it to #"343.15 K"#, the change in temperature will be
#DeltaT = "343.15 K" - "243.15 K" = "50 K"#
The interesting thing to notice here is that #DeltaT# will have the same value if you give the two temperature in degrees Celsius. Remember, to convert from degrees Celsius to Kelvin, you need to add 273.15 to whatever value you have.
#T_"K" = T_(""^@"C") + 273.15 => T_(""^@"C") = T_"K" - 273.15#
This will make #DeltaT# equal to
#DeltaT = underbrace((343.15-273.15)""^@"C")_(color(blue)("final temp")) - overbrace((293.15-273.15)""^@"C")^(color(green)("initial temp"))#
#DeltaT = 70^@"C" - 20^@"C" = 50^@"C"#
So, regardless if you have the temperature expressed in degrees Celsius or in Kelvin, the change in temperature is the same for both temperature scales.
That happens because, by definition, one degree Celsius is equal to 1 Kelvin.
#1^@"C" = "1 K"#
