- 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][1] $$(4,3) \color{#00F}-\begin{pmatrix}1
   \cr1\end{pmatrix}=(3,2),$$ 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.


  [1]: https://math.stackexchange.com/questions/3500269/what-happens-when-a-vector-is-subtracted-from-a-given-point#comment7198833_3500283