It is as cromulent (thanks, MatthewDaly!) to write $$11.40\text{ a.m.} \color{#00F}- 15\text{ min} = 11.25\text{ a.m.}$$ as it is to write $$(4,3) \color{#00F}-\begin{pmatrix}1 \cr1\end{pmatrix}=(3,2),$$ because $“\color{#00F}-”$ here can be thought of (is implicitly defined) as a variant subtraction operation: one whose first argument is a time/Cartesian -point, second argument is a duration/vector, and output is a time/Cartesian -point.
When using the 24-hour clock, I write $$11\;40 \color{#00F}- 15\text{ min} = 11\;25.$$
I avoid writing $$11:40 \color{#00F}- 15\text{ min} = 11:25$$ though, as it is ambiguous whether it is nearing noon or midnight.
ryang
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